I estimate that it has saved us around 3 MegaWatt hours since it was commissioned. In monetary terms, that's roughly £1,000 taken off our electricity bills.

How did I work that out? Well, maths is hard, as Barbie knows, so take all this with a pinch of monosodium glutamate.

Here's a typical month - October 2025:

Yikes! What's going on here?

We use a variable electricity tariff. Prices fluctuate every 30 minutes. At peak times our electricity prices can shoot up to 60p per Kwh. Overnight or when the wind is high, prices can drop to zero. Yes, free electricity! Sometimes the excess in the grid means that prices go negative and we are paid to use electricity. Hurrah!

Our battery knows this. Its Internet connection allows it to download the tariff for the day ahead and plan accordingly. If the electricity prices are cheap, the battery fills up. The battery can decide to discharge when we're using more electricity than solar provides, or it can wait until prices are more expensive after the sun has gone down.

Here's an example, again from October:

In October, about a third of the power stored in the battery came from the sun. About 92% was used by our house with the remainder being sold back to the grid if it was profitable to do so.

By contrast, here's June 2025 - a sunny month in the Northern Hemisphere:

Here, only 12% of the battery charging was done by the grid. 88% was done for free by solar power. But because solar was so plentiful, about 15% of the battery was sold back to the grid.

Maths. Is. HARD!

I've been playing around with various charts, graphs, spreadsheets, modellers, and a bit of calculus. I basically came to the conclusion that the easiest way was to assume I was saving the energy price capped value of a kWh.

That varies from 25p to 35p. If I fudge the numbers just right, it rounds off at an even grand.

It's Payback Time

No-one ever asks what the payback period is of buying a car vs taking public transport. You never see anyone amortising an engagement ring over the length of a marriage. Still, here we are.

We paid £2,700 for the supply, install, and commissioning of our battery.

That means the payback time for the battery will be between 6 and 7 years. If energy prices go up, the payback time goes down. Its capacity is showing no degradation yet and I hope it will provide us with many years of savings before it needs to be repaired or upgraded.

Solar batteries are getting cheaper and their capacity is getting bigger - although not big enough to store all my home's electricity.

If you can afford the upfront costs, it's like pre-paying for a chunk of your energy usage and can help protect you against sudden price rises.

You can sign up to Octopus and get a £50 bill credit if you want to switch to a variable tariff.

On average they generate about 3,800kWh per year. We also use about 3,800kWh of electricity each year. Obviously, we can't use all the power produced over summer and we need to buy power in winter. So here's my question:

How big a battery would we need in order to be completely self-sufficient?

Background

Let's take a look at a typical summer's day. The graph is a little complex, so I'll explain it.

The yellow line shows solar production. It starts shortly after sunrise, peaks at midday, and gradually drops until sunset.

The red line shows how much electricity our home is using. As you can see, there's a large peak about 19:00 when we cook dinner.

The blue line shows how much electricity we draw or export from the grid. From midnight until sunrise we import because the sun isn't shining. Once the sun has risen we're able to power our house and export to our neighbours. When we cook, we draw from the grid and our battery - which is why the evening grid peak is lower than the household use dip.

The CSV of the data looks something like this:

It's fairly trivial to sum both columns and subtract one from the other. That shows either the excess or deficit in solar power for the household.

On that day, the house used 9.7kWh and generated 19.6kWh. I'd need a 9.9kWh battery to store the excess right? Wrong!

Because my usage doesn't track the sun, I'd actually need a 13kWh battery. That's the peak amount of excess electricity I've generated in that one day.

What I want to do is find out what the maximum size battery I would need in order to store all of summer's electricity for use in winter.

Luckily, I have several years of real data to go off! Let's get started!

Disclaimer

This is based on data generated by my home battery. It has probes to measure solar output and grid flow. It is not 100% clock-accurate compared to my solar-panels' internal reporting nor what my smart-meter reports. I estimate a 1-2% deviation, which is good enough for these purposes.

My energy usage isn't representative of anything other than my usage. Your household is probably different. I already have a 4.8kWh battery which changes how and when I use energy.

This doesn't account for gas heating or hot water. We have some electric heaters and taps which increases our electricity usage.

My maths is probably right - but the code is open source, so feel free to check for yourself.

Remember, this is just a bit of fun. There's no practical way to build domestic batteries with this capacity using the technology of 2025.

Code

We tend to start generating more electricity than we use starting in Spring. So I've picked the end of March 2024 to the end of March 2025.

Let's see how big a battery we'd need to store our summer excess for winter. This finds the cumulative difference between each day's energy production and usage:

import os
import pandas as pd

# Load all the CSVs
filepaths = [f for f in os.listdir(".") if f.endswith('.csv')]
df = pd.concat(map(pd.read_csv, filepaths))

# Make sure they're in order
df = df.sort_values("Timestamp")
df = df.reset_index(drop=True)

# Resolution is every 5 minutes, so divide by 12 to get hourly
df["Cumulative_Difference"] = ( (df["Household_(W)"] + df["Solar_(W)"] ).cumsum() ) / 12

# kWh of battery needed
int(df["Cumulative_Difference"].max() / 1000)

## Draw a pretty graph
df.plot(kind="line", x="Local_time", y="Cumulative_Difference", xlabel="Date", ylabel="MWh", xticks=["2024-04-01", "2024-05-01", "2024-05-01", "2024-06-01", "2024-07-01", "2024-08-01", "2024-09-01", "2024-10-01", "2024-11-01", "2024-12-01", "2025-01-01", "2025-02-01", "2025-03-01", "2025-04-01"], legend=False, grid=True, fontsize=15)
plt.show()

The total is 1,068KWh - basically, a MegaWatt-hour of storage.

Here's a quick graph to show how the storage would be used over the year.

As you can see, even in this scenario there are a few days where we'd need to import energy from the grid.

Is this sensible?

Probably not, no. It doesn't account for increased energy use from having an electric car or moving away from gas heating / cooking. As solar panels increase in efficiency, it might be more sensible to replace the panels on my roof, or add some onto a shed.

The environmental impact of creating and storing such huge batteries could also be factored in.

A battery which is only 100% full for a few days probably isn't an efficient design. Using wind, hydro, and other green sources from the grid might be preferable.

But, remember, this is an exercise in wishful thinking.

Is this possible?

Grid-scale batteries exist and they work brilliantly.

But if I wanted my own MegaWatt-hour of battery storage, it would probably cost me between £100k and half-a-million quid.

That doesn't include maintenance, the land, planning permission, and a hundred other things.

But battery prices are falling fast. In the last decade lithium ion battery prices have fallen 90%. With new sodium ion batteries promising an even bigger drop - down to US$10/kWh.

If - and it is a big if - those numbers came to pass, it would probably cost around £8,000 for a domestic battery. Basically the same cost as adding solar panels in the first place.

Domestic solar works - yes, even in the rainy UK! It is relatively cheap, moves energy production as close as possible to energy consumption, reduces bill-shock, and means we don't have endless planning arguments about whether fields should be turned into solar farms.

It is possible that, not too long in the future, every home could also have a 1 MegaWatt-hour battery. They would be able to capture all the excess solar power generated in a year.

There's a bright and sunny future where every home can be solar-self-sufficient.

If you've enjoyed this blog post, please consider switching to Octopus Energy - we both get £50 when you join.

This morning, I took a reading from our generation meter:

19MWh of electricity stolen from the sun and pumped into our home.

That's an average of 3,800 kWh every year. But what does that actually mean?

The UK's Department for Energy Security and Net Zero publishes quarterly reports on energy prices. Its most recent report suggests that a typical domestic consumption is "3,600 kWh a year for electricity".

Ofgem, the energy regulator, has a more detailed consumption breakdown which broadly agrees with DESNZ.

On that basis, our solar panels are doing well! A typical home would generate slightly more than they use.

Net Zero

What is "Net Zero"?

Put simply, net zero refers to the balance between the amount of greenhouse gas (GHG) that's produced and the amount that's removed from the atmosphere. It can be achieved through a combination of emission reduction and emission removal. National Grid

I don't have the ability to remove carbon from the atmosphere⁰ so I have to focus on reducing my emissions¹.

Numbers

All of our panels' generation stats are published online.

Let's take a look at 2024 - the last complete year:

Generation was 3,700 kWh - a little below average. Obviously a bit cloudy!

We try to use as much as we can when it is generated, and we store some electricity in our battery. But we also sell our surplus to the grid so our neighbours can benefit from greener energy.

Here's how much we exported last year, month by month:

Bit of a dip during the disappointing summer, but a total export of 1,500 kWh.

We used a total of (3,700 - 1,500) = 2,200 kWh of solar electricity.

Of course, the sun doesn't provide a lot of energy during winter, and our battery can't always cope with our demand. So we needed to buy electricity from the grid.

We imported 2,300 kWh over 2024.

Quick maths! Our total electricity consumption was 4,500 kWh during the year.

Very roughly, we imported 2,300 and exported 1,500. That means our "net" import was only 800kWh.

There's a slight wrinkle with the calculations though. Our battery is aware that we're on a a dynamic tariff; the price of electricity varies every 30 minutes. If there is surplus electricity (usually overnight) the prices drop and the battery fills up for later use.

In 2024, our battery imported about 990 kWh of cheap electricity (it also exported a negligible amount).

If our battery hadn't been slurping up cheap energy, we would be slightly in surplus; exporting 190 kWh more than we consumed.

So, I'm happy to report that our panels take us most of the way to a personal net zero for domestic electricity consumption.

A Conclusion (of sorts)

The fight against climate change can't be won by individuals. It is a systemic problem which requires wholesale change in politics, industry, and regulation.

But, as a society, we can all do our bit. Get solar panels, install a heat pump, buy more efficient appliances, walk or take public transport, switch to a more sustainable diet, learn about your impact on our world.

More importantly - tell other people what you're doing!

Speak to your friends and neighbours. Shout about being more environmentally conscious on social media. Talk to your local and national politicians - explain to them why climate change is a personal priority. Write in favour of solar and wind farms being installed near you. Don't be silent. Don't be complicit in the desecration of our planet.

Bonus referral link

The import and export data is available via Octopus Energy's excellent API. They also have smart tariffs suitable for people with solar and / or batteries. Join Octopus Energy today and we both get £50.

That's a lot of (expensive) gear! What does it mean in terms of energy savings?

Over the last 12 months we have:

• 4,000 kWh generated by the solar panels.
• 1,200 kWh purchased from the grid.
• 1,200 kWh sold to the grid.
• 1,300 kWh discharged from the battery.

(Data taken from various APIs and rounded to make life easier).

It's important to note that the battery doesn't only charge from the panels. Because we have a a smart electricity tariff, it will fill up from the grid when electricity prices are negative and empty when they are expensive.

Calculations

Generation - Export = Solar Use

4,000 - 1,200 = 2,800kWh

So we directly use about 70% of what we generate. Some of that is used immediately, and the rest goes into the battery for later use.

Using the battery's API, I was able to call up the statistics for each month. For example, in June, we only paid for 10% of our electricity!

I got the data for each month, these aren't quite as precise as I'd like, but they're good enough for modelling.

Domestic Electricity use kWh by Source per Month:

Wow! Averaged across the entire year, we've cut our electricity bill by about ⅔rds!

Across the year, we sell back as much electricity as we buy. We are net zero!

For reference, the average UK home uses about 3,500kWh. We're a little above average - probably due to working from home.

Cost

Electricity prices have been wild in the last 12 months. The dynamic tariff means we can buy cheap energy but use it when it is expensive. So it is hard to say with any accuracy how much money we've saved. I'm going to say very roughly that electricity costs 24p/kWh and we sell it for 12p/kWh.

( (Solar Used + Battery Used) x 0.24 ) + (Electricity Sold x 0.12) = Total Savings

( (1,280 + 1,243) x 0.24 ) + ( 1,200 x 0.12 ) = £750

The cost of installing solar and a battery will vary depending on where you are, what local incentives there are, how complicated your roof and existing electrics are, and a whole host of other factors. Payback depends on your usage patterns and energy prices.

We estimate our payback period to be between 6-8 years.

Obligatory Referral Links!

If you'd like a dynamic electricity tariff, or want an API to monitor your own use, or need to sell electricity back to the grid - join Octopus Energy today and we both get £50.

Well, after less than 10 months, the battery has given us 1MWh.

To put that in to context, the average UK household uses about 3MWh per year. So (again, very roughly) over a third of our electricity use this year has come from the battery.

But where does the battery get its energy from? We have two sources.

First is solar. When the sun is shining, our solar panels produce electricity. That flows down from our roof and into our mains wiring where it is used by the home. If we are using less electricity than is being produced, the electricity flows into the local grid and we get paid for selling our surplus.

Our battery has sensors attached to the grid connection. When it detects surplus generation, it starts charging. By constantly monitoring our overproduction, it can charge up with free solar power.

But the sun doesn't always shine (ain't that the truth!) so there are days when our solar production is less than our usage.

In these cases, the battery charges from the electricity grid. We have a smart tariff which changes price every 30 minutes. The battery knows the day's prices and can predict our daily usage. If it can see that electricity is cheap at 3am and expensive at 4pm, then it will charge up during the early hours of the day and discharge at peak time.

The battery occasionally sits idle. Mostly when it has fully charged but knows an expensive period is coming up later.

What does that mean for money?

Well... it's complicated! When the battery charges from solar, is the electricity free? No! If we were to sell that surplus electricity to the grid, we would be paid 15p/kWh.

When the battery charges from the grid, is the electricity expensive? No! Because we are on a dynamic tariff, we occasionally get paid to use electricity! Our provider has paid us up to 5p/kWh to charge!

When the battery discharges, how much does it save us? Again, complicated! Because we're on a dynamic tariff our prices change every 30 minutes. Sometimes the rates are as high as £1/kWh, other times they're 1p/kWh. Generally speaking, the battery only discharges if the price of use is higher than the cost of acquisition.

So... I've fudged the figures! For the first year of operation, energy prices have been high. Based on a back-of-a-fag-packet calculation, I reckon the battery saves us an average of about 31p/kWh. Call it about £360 per year in savings.

That gives us a payback time of about 8 years.

Of course, if electricity prices spike, payback will be quicker. If they crater, it'll take longer. If we switch to electrical heating or get an electric car, the savings will be greater.

Domestic battery technology is still a bit of a tough sell. The batteries are large and their fans are noisy. The cost of materials and installation is high and their capacity is relatively small. But the technology behind them is sound. With a dynamic energy price tariff, they're one of the best way to reduce utility bills.

Obligatory referral link

Join Octopus energy and we both get £50. They have regular and dynamic tariffs, and a pretty cool GraphQL API.

Four years later and our generation meter shows they have produced 15,480kWh of electricity.

That's a rather impressive 3,870kWh per year.

This is what the average UK household uses in electricity:

Our roof is non-optimal. We have an East/West split rather than the more usual South-facing panels. We have some big trees near us. And we live in a rainy, cloudy, overcast region of London.

We still produce more than the average household consumes!

Depending on the Carbon Intensity of the UK grid, we've saved about 3 tonnes of CO₂.

Imagine if we could put solar on the roof of every house in the country. Domestic consumption from the grid would fall to zero on sunny days. Even in winter it would dramatically reduce usage. If you can add a battery and smart meter then domestic costs would plummet.

Money

Let's talk about money for a moment. I don't know how much it will cost you to put solar panels up on your roof. You might need lots of scaffolding, your wiring and consumer unit may need an upgrade, there might be bats in your loft. Get a couple of quotes and find out for yourself.

But I can talk about how much money solar panels can save.

Over the last 4 years, UK electricity prices have fluctuated considerably. Somewhere between 20p/kWh and 30p/kWh. That means those 15,480kWh are worth somewhere between £3,000 and £4,600.

Of course, not all of that electricity gets consumed by us. Some of it flows back to the grid. As a rough rule of thumb, most systems seem to export half of what they produce.

Octopus Energy (join and we both get £50) pay 15p/kWh on export.

So let's say 50% is used, saving 25p/kWh, and 50% is sold at 15p/kWh. That means we have profited by roughly £3,000 over 4 years.

Data

Every day at sunset, I automatically publish data straight from the solar inverter. You can grab the data from GitLab. It will show you real data, changing every 5 minutes, and could be useful if you're modelling something.

My previous dataset from the last house we lived in has been used in a number of academic studies.

Solar panels are great for geeks! I deliberately got an Inverter with a local API & Ethernet connection.

Should I get solar panel?

Photovoltaic cells are the ultimate in "boring magic". Stick them on your roof and forget them. Year after year they'll happily sit inert, chugging down photons and spitting out electrons. The rain washes them clean. The warranties are generous. The technology behind them is stable and well understood. All you have to do is watch your electricity bills fall.

If you've enjoyed this post, consider joining Octopus Energy using my referral link - they have great tariffs for solar panels.

So our winter electricity bills must be massive, right?

Nope.

The normal cost per kWh is 28.5p (including VAT). We're paying less than half that - 12.4p per kWh.

This is thanks to two things - a smart tariff and a home battery.

The Octopus smart tariff charges us a variable amount throughout the day. Every 30 minutes the prices change to reflect the demands on the grid. During peak times, it can go as high as £1/kWh. That's a good incentive not to run the tumble-dryer at the same time as the rest of the country is cooking dinner!

During quieter times, the price of electricity drops - there isn't much demand at 3AM so prices fall. Sometimes they fall to zero. Other times, they fall into negative territory and we get paid to use electricity.

Now, that's all well and good, but most people don't want to shift their consumption habits. The dishwasher goes on when it is full and dinner is cooked before Coronation Street starts. That's where the battery comes in.

We have a 4.8kWh battery. It is hooked up to the Internet and knows what our energy prices are minute-to-minute. When electricity is cheap, it charges up from the grid. When electricity is expensive, it discharges into our home. If we boil the kettle at 7pm, the sensors on the battery detect that we're using expensive electricity and starts outputting stored electricity.

Essentially, we don't have to alter our lifestyle at all. Here's a typical December day. The graph is quite complicated, so let me step you through it.

The bottom graph shows how expensive it is to buy electricity throughout the day. As you can see, there is a peak in the early evening when electricity becomes expensive.

The top graph has two interesting lines on it. The purple line shows how much electricity we're drawing from the grid, the blue line shows what the battery is doing. Early in the morning electricity is cheap - you can see the purple line rising as the blue line falls. That shows the battery is charging. You will notice that it only charges at the cheapest possible times.

In the evening, you can see the purple line dip to zero and the blue line rise. That shows the battery is discharging into our home and there in no electricity being purchased from the grid. There's a similar dip at about 0830 when there's a little spike in price. Clever battery!

I want to stress that is is all automated. I don't have to do a single thing. The battery speaks directly to my electricity provider to get the half-hourly costs. The battery can predict what our usage will be, but keeps most of the electricity for the expensive times of day. Our smart meter sends our usage back to the energy company automatically.

Savings

Against a normal tariff of 28.5p/kWh, I'm paying 12.4p/kWh. That's a saving of 16.1p/kWh.

The bill above shows 320kWh per month, which means a saving of £51 from the electricity I buy. That's approximately a 55% discount.

We've had that battery since August, so about 5 months. In that time it has saved us approximately 500kWh. We only moved onto the smart tariff a few months ago, so work out the savings there is complex - but I estimate it's about £130.

December is a high use month (lots of lights on and oven cooking). During summer, the battery mostly fills up with free solar power. It is hard to predict exactly what we'll save in a year, but it should easily shave 50% off our electricity bills.

Cost

But, of course, there's no such thing as a free lunch. Our 4.8kWh battery cost about £2,700 to supply and install. That's a large chunk of change. Based on our current projections, its payback period should be about 7 years. Of course, if electricity prices rise significantly, the payback period will shorten.

Solar panels are also expensive to install - between £4,000 and £12,000 depending on your property and how complex your roof is. They mean we pay virtually nothing for electricity in spring and summer. Again, the payback period is under a decade.

We can also sell our excess solar back to the grid. In theory we could also buy cheap electricity in the morning, store it in the battery, and then sell it back at peak times. In practice it isn't worth it; the cost of buying electricity at peak is higher than the price we could sell it for. So it makes sense to use the power rather than selling it.

If you can afford the large up-front capital costs, solar + battery allows you to make massive savings with a dynamic tariff. In times of solar excess, we pay close to nothing per kWh. In winter, we shift our consumption to pay at the cheap rate.

Effectively, it's like pre-purchasing all your electricity for the next decade.

Final thoughts

There's no doubt that the cost makes this prohibitive to many people. Ideally, the state should be mandating that all new homes have solar panels and space for optional batteries. We also need V2G (Vehicle to Grid) to allow electric cars to act as home batteries.

But there's no doubt that these technologies actually work! Yes, solar works in rainy London. And, yes, even fairly small batteries can make a significant difference in winter. We're on the cusp of a domestic energy revolution. When coupled with a smart tariff, it means people don't have to change the way they behave in order to save energy.

Well, not quite.

On Christmas Eve, my electricity company emailed me to say that I would have several hours of free electricity. They would charge me £0.00 per kWh. More than that, at a few specific times they would pay me for my electricity use!

Here's the graph of my half-hourly prices:

Most factories and heavy industrial plants weren't running the day before Christmas. UK power usage spikes when everyone boils a kettle at the end of a football match or other similar event - but there was nothing so momentous happening at 3AM. So supply outstripped demand.

Anyone with a smart-meter could have been paid to charge their car, run their tumble dryer, or stay up until the wee hours playing on their console.

And was it nuclear power which did this? No.

As shown on the live grid tracker about two-thirds of the day's electricity came from renewables. It was pretty overcast, and our solar panels barely made 1kWh.

It wasn't mined uranium which gave us power which literally had to be given away; about 62% of the electricity came from wind.

At this point, the nuclear lobby will start whinging about subsidies (both nukes and renewables are generously subsidised) and how wind can't provide a base load (which is fair). But although sticking a bunch of turbines in costal waters is an engineering marvel - it's pretty cheap compared to building and maintaining a nuclear power station.

Wind - and other renewables - have done what nuclear couldn't. They have provided such an abundance of electricity that consumers are paid to use it.

History and the Future

It's worth looking at the original quote from 1954 about electricity becoming too cheap to meter:

As well as nuclear, he talks about "photosynthesis". Well, the UK now has 15.6 GW of solar capacity across 1,430,994 installations. A small part of that is my solar panels!

The UK also has around 27GW of wind capcity installed.

It is entirely possible that the UK will have generated the majority of 2023's electricity from renewables.

Because home appliances are increasingly efficient, domestic energy use is falling - it's down 19% since 2010. Electricity use by domestic properties was about 96.2 TWh in 2022 and 135 TWh was generated by renewables.

Yes, electricity is fungible, but you can convincingly make the case that every home in the UK was powered by renewables.

Solar panels don't work at night, and wind-turbines don't work when there's no wind. We'll always need something to be able to provide a base-load of electricity. That might be nuclear, or fossil fuels, or it might be storage from the excess power from renewables.

Sadly, the world is still filled with war, famine, and disease. But, for a few moments on a winter's evening, wind power genuinely became too cheap to meter.

Shameless Plug

If you want to move to a time-of-day electricity tariff, you can join Octopus Energy - if you use that link, we both get £50 bill credit.

Background

After installing solar panels, a smart electricity meter, and a solar battery - the next obvious step was a smart energy tariff.

Octopus (join and we both get £50) have an "Agile" tariff. Unlike a normal tariff - with a set price for electricity - this tariff fluctuates every 30 minutes. Prices depend on wholesale costs which means they can go negative. That's right, you can get paid to soak up excess power.

Of course, they can also spike considerably. Unlike the failed Texas experiment, here the maximum price is capped at £1/kWh.

Every day at about 1600, the next day's prices are published on Octopus's website. And they're also made available via a simple REST API.

So, it's relatively simple to generate a line graph and display it on the eInk screen.

Code

(You can treat this code as MIT Licenced if that makes you happy.)

Calling the API for the half-houly prices is:

$url = "https://{$API_KEY}:@api.octopus.energy/v1/products/" .
       "AGILE-FLEX-22-11-25/electricity-tariffs/E-1R-AGILE-FLEX-22-11-25-C/standard-unit-rates/";

Your API_KEY is unique - and you'll need to check which tariff you're on.

The data is retrieved as JSON and converted:

$content = file_get_contents($url);
$data = json_decode($content);

The JSON is full of entries like this:

"results": [
{
  "value_exc_vat": 13.6,
  "value_inc_vat": 14.28,
  "valid_from": "2023-11-01T22:30:00Z",
  "valid_to": "2023-11-01T23:00:00Z",
  "payment_method": null
},
{
  "value_exc_vat": 18.4,
  "value_inc_vat": 19.32,
  "valid_from": "2023-11-01T22:00:00Z",
  "valid_to": "2023-11-01T22:30:00Z",
  "payment_method": null
},

They're newest first, so need to be reversed:

$tariffs = array_reverse( $data->results );

Then it's a case of looping through them and grabbing today's data:

$userTimeZone = new DateTimeZone('Europe/London');
$now = new DateTime('now', $userTimeZone);

$nowPosition = 0;
$datay = array();
$datax = array();

foreach ( $tariffs as $tariff ) {
    $dateStringFrom = $tariff->valid_from;
    $dateStringTo   = $tariff->valid_to;
    $dateTimeFrom = new DateTime($dateStringFrom, new DateTimeZone('UTC'));
    $dateTimeTo   = new DateTime($dateStringTo,   new DateTimeZone('UTC'));

    if ($now >= $dateTimeFrom && $now <= $dateTimeTo) {
        $costNow = $roundedInteger = (int)round( $tariff->value_inc_vat );
        $hour   = intval( $dateTimeFrom->format('G') ); //  No leading 0
        $minute = intval( $dateTimeFrom->format('i') );
        $offset = ($minute == 0) ? 0 : (($minute == 30) ? 1 : null);
        $nowPosition = (2 * $hour) + $offset + 0.5;
        $until = $dateTimeTo->format('H:i');
    }

    if ($dateTimeFrom->format('Y-m-d') == $now->format('Y-m-d')) {

        $datax[] = $dateTimeFrom->format("H:i");
        $cost = $roundedInteger = (int)round( $tariff->value_inc_vat );
        $datay[] = $cost;
    }   
}

Drawing the graph uses the venerable JPGraph:

$path = 'jpgraph/';
set_include_path(get_include_path() . PATH_SEPARATOR . $path);
require_once ('jpgraph/jpgraph.php');
require_once ('jpgraph/jpgraph_line.php');
require_once ('jpgraph/jpgraph_plotline.php');

// Size of graph
$width  = 600;
$height = 600;

// Setup the graph
$graph = new Graph($width,$height);
$graph->SetScale("intlin");
$graph->SetMargin(35,0,45,20); // L R T B

$graph->SetUserFont('dejavu/DejaVuSansMono.ttf');
$graph->title->SetFont(FF_USERFONT,FS_NORMAL,25);

$graph->SetBox(false);

$graph->title->Set( $now->format('l') . "'s Electricity Prices\n" . $costNow . "p / kWh until {$until}" );
$graph->title->SetColor('#000');

$graph->ygrid->Show(true);
$graph->xgrid->Show(true);

$graph->xaxis->SetTickLabels( $datax );

$graph->xaxis->SetColor('#000');
$graph->yaxis->SetColor('#000');

$graph->xaxis->SetFont(FF_USERFONT, FS_NORMAL, 10);
$graph->yaxis->SetFont(FF_USERFONT, FS_NORMAL, 14); 

// Just let the maximum be autoscaled
$graph->yaxis->scale->SetAutoMin(0); 

// Only show up until 23:00
$graph->xaxis->scale->SetAutoMax(46);

$graph->xaxis->SetTextLabelInterval(2); 
$graph->SetTickDensity(TICKD_DENSE, TICKD_DENSE);  

// Create the line plot
$p1 = new LinePlot($datay);
$graph->Add($p1);
$p1->SetStepStyle();
$p1->SetColor('#000');

//                 Direction, position,     colour@alpha, width
$l1 = new PlotLine(VERTICAL,  $nowPosition, 'black@.8',   13);
// Add vertical highlight line to the plot
$graph->AddLine($l1);

// Output line
$graph->Stroke();

Next steps

I dunno? Add some details about carbon emissions? Battery stats? Let me know what you think in the comments.

We all know that solar panels' efficiency wilts in the heat, but do they get a tan work standing in the English rain?

At the beginning of August we installed a 4.8kWh solar battery to supplement our 5kW of solar panels.

The battery provides a CSV of readings taken every 15 minutes. It measures solar power, household usage, and battery usage. August, looked like this:

Not massively helpful. But, with a little bit of 🐍 Python and 🐼 Pandas, I worked out the following:

🏠 Our home used 290kWh

We're tracking pretty close to the UK average of about 10kWh per day. Our average of 9.4kWh each day is perhaps slightly higher than normal for a 2-person household. But we work from home regularly and have a lot of hungry smarthome gadgets.

🔌 Only 15% of our electricity came from the grid

The sun doesn't shine at night. Duh! But the battery usually provides most of our power after sunset. The battery can only discharge at a maximum of 2.4kW, I think. So if we use the electric shower, oven, or other high power appliances, then we draw from the grid.

So, what did that 44kWh cost us?

💷 Normally, we'd pay £90 for August's electricity. We only paid £14!

The price you pay for electricity depends on where you are in the UK and what tariff you're on. With a mix of solar and battery, we cut our August bill by 85%.

But what's the mix between solar direct and solar delayed?

🌞 Solar gave us 46% of our electricity

About 135kWh of our month electricity needs was met directly from solar. That means photons hit the panels, they bounced down into the inverter, and then straight into the wires in our walls, where they were gobbled up by laptops, TVs, and toasters.

That just leaves the battery…

🔋 Battery storage gave us 39% of our electricity

We used about 113kWh from stored solar. An average of 3.6kWh per day. Perhaps that means our 4.8kWh battery is over specced? I'm not so sure. Some days we use over 100% of our solar battery capacity.

Of course, not all of the solar power can get used or stored. Once the battery is full, that electricity has to go somewhere…

🔙 We sold 140kWh of solar back to the grid

Our solar power feeds into our local grid for our neighbours to use. We sell the electricity at market rates - which change every 30 minutes. This made us £13.

📈 Total cost for August's electricity? £1.

Yup! For the whole month of August, our electricity bill was £1.

(Plus the standing charge, of course!)

Disclaimer

OK, time for a little bit of a sanity check.

Firstly, these data are drawn directly from the battery. It has clamps over our import and export wires to monitor what the household is doing. These are broadly accurate - I estimate less than 2% different from what our smart meter and inverter report.

Secondly, the battery groups up the stats every 15 minutes. So, again, that's likely to introduce some errors into the data.

Thirdly, prices for both import and export can vary massively. Our export price in particular varies depending on demand.

Fourthly, these data were gathered in South-East London on an East / West split solar site. Your panels will be in a different location and will perform differently.

Fifthly, the price of panels and battery storage is high. If you can afford the up-front capital costs of an installation, I think it makes sense to do so. The payback period is usually under 10 years. But can be considerably shorter during a time of rising energy costs.

I want more stats!

Every day at sunset, my solar panels publish their generation stats to GitLab. You can download all the data from 2020 and see how much solar generation we've had.

If you need more, I published 5 years of minute-by-minute solar generation as Open Data from our previous house. This dataset has been cited in several academic papers.

I'm considering whether to release my daily usage statistics. At the moment, it feels a little invasive. You can tell when I put the kettle on in the morning, see when I load a tumble-dryer, and calculate just how long I use the oven for. Perhaps you can even analyse the overnight fluctuations and work out what model of fridge I have. I don't think you can tell what video content I'm watching because it's hidden in the noise of my other appliances. But you could probably tell if I was home or not.

Here's a typical daily graph. What do you think you can figure out from this?

So I think I'll release it in a year's time. That's a decent balance between openness and privacy.

I hope you've found this blog post useful. If you have, you can support me by:

• Switching to Octopus Energy - if you join, we both get £50. They do dynamic pricing for import and export. And, even better, they have an API so you can query your energy usage.
• Supporting OpenBenches - it's a crowdsourced site of memorial benches run by me and my wife.
• You can also buy me a book to read.

Thanks!

We recently got a 4.8kWh solar battery installed. Batteries are still somewhat complex beasties. In order to prevent damage to the internal structure, a BMS (Battery Management System) ensures that each cell in the battery gets a fair share of wear and tear. One side effect of this is that our battery never dips below 10% charge.

So our usable capacity is 90% of 4.8kWh - which is 4.32kWh.

The other day my solar panels sent me a message on Mastodon saying:

5.9kWh? Oh buggeration, my shonky Python code has messed up. Right? Wrong!

Here's what the battery did over a 24 hour period:

We started at midnight with a battery about 30% full. Throughout the night it discharged into our circuits, getting down to 10%.

Come sunrise, the battery started charging. It had a short discharge as we turned on temporary loads like the kettle.

By about 1200 it was fully charged - only to start discharging again when we started cooking in lunch, using the dishwasher, etc. Luckily there was enough residual sunshine in the afternoon to top it up to 100% again.

Then, throughout the afternoon and evening, the battery did its job and powered our WFH laptops, TVs, game consoles, electric showers, ovens, etc. Here's what our energy use looked like:

And that's how our 4.32kWh usable became 5.9kWh actual in a single day.

The battery's maximum charge/discharge rate is 2.4kW. So theoretically we could do a charge/discharge cycle 6 times per day and use 28.8kWh - but that is unlikely to make economic sense unless electricity costs fluctuated wildly throughout the day.

So, depending on your usage patterns and the variability of energy prices, you don't necessarily need a battery spec'ed to 100% of your average daily use.

That means... GRAPHS AND STATISTICS!

Our battery was commissioned just after midday. This graph shows:

• 1630 - it was fully charged.
• 2000 - the sun had set and the battery started discharging into our home.
• 0700 the next day - the sun rose and started generating electricity, so the battery stopped discharging.
• 0900 - solar was generating more power than the house needed, so the battery started charging.
• 1400 - the battery was fully charged.
• 1830 - we started cooking dinner in the oven and the battery started discharging.

Let's see how that maps on to our energy usage. This is what a typical WFH day looks like:

To be clear, this is how much electricity I used during the 24 hour period. This graph doesn't care if the electricity came from solar, the battery, or the grid.

Our total energy mix for the day was:

The majority of our electricity use today came from yesterday's solar!

And what does that do for our electricity bills? We use Octopus Energy (join and we both get £50) who take smart-meter readings every 30 minutes. Here's the graph they provide for the electricity we bought:

It is hard to perfectly calibrate a battery to monitor the precise energy flow - but that's pretty bloody close! Ideally that would be reading 0 Watts overnight as the battery supplies the house. But I can live with that tiny trickle!

In total, we drew 433 Watts from the grid over 24 hours. That cost us £0.09! The battery saved us about £0.75 that day.

We're lucky to be on a fixed-rate tariff where the energy prices are low. If we were on a modern tariff, the UK's price cap is 30p/kWh. So we'd have paid £0.13 for the grid electricity, and saved £1.01 using the battery.

Big Graph Showing Everything

This is the view of our electricity use as seen by the battery's sensors:

The red shows what we actually used - and you can see the battery in blue discharging to match demand. The purple line shows what the grid was doing - we exported about 5.8kWh of electricity.

Finally, the yellow shows the solar being generated. As described above, the battery has a sensor to detect the current flowing through the wires. How does that match with what our inverter is reading?

They're almost an exact match!

Conclusion

Solar batteries have a large up-front cost. But they work. On a fairly wet and dreary summer day, they performed perfectly. As we move into winter, we'll switch to a dynamic electricity tariff. That will allow us to charge the battery when power is cheap and then use it when prices are high. So expect another blog post in 4 months' time describing if that has worked as expected!

Seven years ago, we installed a solar battery. It was part of an experimental project which looked at creating a community power-grid, so it came at a subsidised price.

As I explained to BBC Click, the 2kWh capacity was reasonable - but I expected the future would bring higher capacity, cheaper costs, and smaller sizes⁰.

But, after 7 years the battery was starting to show its age. The little Raspberry Pi inside it needed more frequent reboots, the fans were grinding a bit, and it would occasionally drop off our network.

So Moixa made me an offer I couldn't refuse. Upgrade the battery to the newest 4.8kWh model. Total cost for supply and installation? £2,700.

Ooof! That's a lot.

We did the maths. The previous subsidised battery had paid for itself over the 7 years. And with electricity prices rising, it made sense to be able to use more of our home-grown supply. So we went for it. Also, I like buying new gadgets.

Installation

Someone comes along with a big drill and lots of electrical flex. The whole thing is mounted on the wall like this:

And then covered up like so:

The old battery was sent to be recycled.

A few minutes after it was commissioned, the website was showing power flowing from the solar and going into our battery and home - with enough left over to sell to our neighbours.

The Good

It can store nearly 5kWh of sunshine. That's about £1.60 worth of electrons at today's capped prices.

Smart profiles - it can monitor how much energy the house is using and charge / discharge as necessary. Or, it can look at realtime energy prices and make a decision about whether to charge. We'll move onto a smart-tariff soon and it will charge up from the grid when prices are low and allow us to sell our excess when prices are high.

It can connect via WiFi and Ethernet. Moixa supply some homeplugs for the Ethernet connection. That allows it to look up energy prices and means Moixa can monitor the battery's health.

It has a modular design. Inside are two US2000 batteries. They're of a fairly standard design and - in theory - could be swapped out for something bigger and cheaper in the future.

Oh, also, there's an app! So I can check what the battery is doing while on the go. A bit silly, but lets me know if the battery is full - which might mean I decided to switch on a high-energy device like a tumble-dryer.

The Bad

It's a big brute! There's been a bit of effort made to soften its industrial appearance. But there's no getting away from the fact it is a big box with wires coming out of it. It needs a bit of airflow around it for the fans, and a sturdy wall to be mounted on.

It's a bit noisy. There's the hum of the inverter, the whirr of the fans, and the occasional click of the relays. Unlike a gas boiler, it is on constantly. Moixa recommend installing it out of the way if possible. Ours is in the porch, so we only notice it when entering or leaving the house.

The information screen doesn't say what the state-of-charge is. All you get is whether it is connected to the Internet and whether it is charging, discharging, or idle. A few more stats on it might have been nice.

If the power goes out, so does the battery. The UK has strict rules on "islanding". If the electricity went out but the battery kept feeding back into the grid it could injure people working on the wires. So it can't be used as an Uninterruptible Power Supply.

The battery's network connection doesn't expose any open ports. I mean, I suppose that's good for security - but I wanted to play! The website is great - but I don't think it offers an API.

Economics

*sigh* Money, eh?

What's the return on investment for your new kitchen? Or for getting your garden landscaped? Sometimes it is nice to have cool things. And, as we head further into a climate emergency, being able to reduce our personal dependency on polluting forms of electricity is a good thing.

But let's talk money anyway!

On the first day, the battery charged up to 95% on solar. At about 8pm, it started discharging. At 8am the next morning it was down to about 35%.

60% of 4.8kWh is 2.88kWh. Divided by 12 hours is 240 Watts. That's how much our house uses over night. We paid £0.00 for electricity during those hours.

The average price of electricity is about £0.35 per kWh - so we saved £1.01.

£2,700 / £1.01 / 365 days = 7 years, 4 months.

But… of course, it is a little more complicated than that. Here's what our solar panels do on a rainy winter's day:

Hardly any solar generated, not enough to even half fill the battery. So the system is useless, right?

Nope!

We're moving to a smart-tariff from Octopus (join and we both get £50). When electricity prices are cheap, the battery will charge from the grid. When electricity prices are high, the battery will discharge. We effectively become small-time arbitragers. We buy low and sell high.

And, if electricity prices go higher, our payback time will be shorter.

Come back to this blog in the year 2030 and I'll tell you how the battery performed.

Verdict

Solar batteries are expensive. There's no VAT to pay if you get them installed at the same time as your solar panels - but the Government doesn't seem keen on making it cheaper for people to retrofit energy-saving measures.

Battery tech is still in its relative infancy. We don't have Moore's Law increasing capacity and reducing prices. So for now we're stuck with big expensive boxes.

If you have the space and money for one, I think solar batteries are a reasonable investment. They're the ultimate in "think global, act local" when it comes to electricity.

Here's how those three years look, according to our smart meter.

Quick maths!

The difference between the generation and the export is our solar consumption: 4,604kWh over 3 years.

So we directly use about 38% of the solar we generate. The remaining 62% gets sold back to the grid. I thought our consumption would be higher; we work from home during daylight hours and we have a modest 2kWh battery. But it is exactly in line with what we were seeing after one year.

Our solar consumption and grid import is the total electricity that we used: 12,125kWh over 3 years.

The average UK home uses around 3,800kWh of electricity per year. We're a bit higher than that. Despite all our energy efficiency measures, we use about 4,000kWh per year.

That means, on average, solar produced 99% of our electricity needs across three years!

You can read more about our solar installation if you'd like to understand the number of panels, details of the inverter, and other technical gubbins.

Money Money Money

People always ask how much the panels cost. That's a difficult question to answer. Every house is different. We have a 3 storey house, which meant extra costs for scaffolding. Our roof was in decent shape, so didn't need reinforcement - and we sold the tiles which were removed. The panels we wanted weren't in stock, so we got a slight discount. Etc etc etc. Basically, go get a couple of quotes and see how much it will cost in your specific circumstances.

The next question is about how much money it has saved us. Again, that's slightly complicated. Energy prices have been "interesting" in the UK. At an average price of 30p/kWh, the panels have saved us about £1,400 over the three years.

But! We also sell the excess electricity. We get paid a variable amount depending on demand - usually between 15p to 30p per kWh. By my calculations, we've sold about £1,500 worth of electricity over 3 years.

Let's call it a total saving of £1,000 per year. If electricity prices go higher, we'll save more. If they drop, we'll save less.

Open Data

Every day, a robot uploads our solar generation data to GitLab where it is freely available for research purposes. The solar stats from our previous home are also available.

☀️

Solar "just works". Yes, even in dreary, rainy, cloudy, foggy UK. Yes, even if your roof doesn't face the optimal direction. Yes, there is a large up-front cost. But even with the current technology, solar is a completely viable way to generate your own electricity and save money.

But it doesn't need to be this way. Exposing users to the immense variability in pricing is probably too dangerous - as seen in Texas recently. Imagine if your electricity provider could say "hey, prices are going to be high tonight. We'll pay you to use less electricity!"

Well, that's what my energy provider are doing! Here's an email I received from them:

Join Octopus and we both get £50!

The UK is experimenting with flexible energy pricing. This is made possible by the large-scale rollout of smart meters. They measure electricity consumption and report it back at up-to half-hourly intervals⁰.

My solar panels already take advantage of this capability. I get paid the half-houly market-price for every kWh of sunshine I export. Octopus publish their price so, in theory, I could tell my battery to store energy at midday when it is cheaper and then sell back to the grid when it is more expensive.

Now, there's just one catch with this offer. The reduction is based on your average usage at that time. If you usually have your electric oven on between 1700-1800 then you'll be able to make some big savings. But if you're already fairly efficient, it is less likely you'll be able to take advantage of this.

The day before the experiment, this was our half-hourly usage:

So, if we were to turn off all our electricity we might be given about £1.95 as a reward. Not nothing, but perhaps not worth sitting in the dark for an hour?

But, in the spirit of experimentation, we tried it! Lights off, TV & games consoles off, chargers unplugged, no cooking or cups of tea. We made our own entertainment¹.

What we earned & what we learned

Using our home monitoring software, here's what happened to our electricity use for that hour:

As you can see, at 1700 we dropped our usage significantly. The hours before and after, we were using about 900Wh. But from 1700-1800 we were down to about 184Wh.

That's about 20% of our usual use for that hour. Our usual cost at that time of day would have been around £0.20 for the hour. Instead, we only used about 4 pence-worth of electricity.

A few days later, we got this message from our electricity provider:

OK! They base the average over the last few weeks, so it isn't surprising that their numbers differ slightly from mine.

So, how much of a reward do they pay us?

I have no idea why they do this in points. Presumably I might be able to exchange the points for a chance to win a prize? Seems silly to me; just use money.

And then, of course, the inevitable "nudge":

I'm not sure how I feel about that sort of nudge. I think we did everything we reasonably could. I'm not a big fan of that sort of competition.

Was it a hardship for us to turn out the lights for an hour? No. But there is a slightly worrying idea that people might feel pressure to turn off vital things like life-saving equipment, freezers, or alarms.

Will people change their behaviour for a couple of quid? That remains to be seen. Should people have to change their behaviour? That's a more complex question. Janet Gunter has a blog post which explores the limitations of this approach.

The idea of a personal carbon footprint has despicable origins - but when scaled up to a population level, it can be a useful way to phrase a problem.

If people merely shift their electricity consumption to an hour before or after the load shedding period, does it have a meaningful impact?

Anyway, I think it is useful to do experiments like this. Perhaps thousands of people will find it useful and it'll scale up to millions of people. Perhaps it won't make a difference at all. But the only way we can find out is by experimenting. And I'm eager to see what happens!

If you're interested in trying this, join Octopus and we both get £50!

How did the rest of you do?

WNJ85

@Will L-J

The figures are in for #OctopusEnergy #SavingSessions and am pretty happy with the outcome considering didn’t go to extremes like some! Popped the #ASHP into ‘Holiday Mode’ for the hour, delayed dinner prep till after 18:00 and didn’t plug the #BEV in to charge straight after getting home as one would normally if needed charge!

---

wishy

@Steve Scott

@Edent 48% cut aparrently. Not bad for a test run without changing much

---

hd

@Hermit Dave

Took part in 1st #SavingSessions by #NationalGrid & #OctopusEnergy. 83% reduction in electricity usage over an hour compared to average of previous 10 days.

---

mttkng

@Matt King 🍔

Got the results of my #OctopusEnergy #savingSessions from earlier this week. I cut down usage in that hour by 89% earning a bonus of £1.43. Apparently I saved 59% more than my neighbours. Useless bunch.

---

sfscotland

@S Fraser

@Edent I'd completely forgotten to check until I saw your results. I saved the equivalent of £4.61 with a 91% reduction in use during that hour.

---

There were some great talks about the scale of the problem - both in terms of the number of properties which need improving and the cost of retrofitting. A bunch of teams showed off some impressive demos which aimed to tackle the issues.

My demo was reasonably simple and (I thought) quite effective. It displayed a satellite view of a street in the local area. By clicking on rooftops, virtual solar panels are added to properties. Click a button and it shows your community how much CO₂ they will save, and how much money they will make.

The demo worked (phew!) and received good feedback. For something built in a couple of hours, I'm pretty pleased with it. The way it works is...

• Draw an interactive map using Leaflet.
• Use ESRI's satellite images.
• A bit of JavaScript to detect click and draw a cool solar panel icon.
• The EU has a great and free API for calculating solar irradiance. So this sends a request for each panel giving its latitude and longitude.
  • TODO! At the moment, it assumes all panels face south. A future version should let each icon be rotated to the correct degree.
  • TODO! Each icon represents 4kWp of solar. A future version should let each panel size be defined individually.
• Use CORS Anywhere because making JS requests is hard now.
• Do maths to work out the total installed capacity.
• Do maths to calculate the CO₂ value.
  • TODO! This is calculated as 0.256 Kg/kWh. I found that figure on a website. Might not be accurate for the UK.
• Do maths to calculate the cost savings.
  • TODO! It assumes 50% export at £0.25/kWh and 50% reduction in use at £0.50/kWh. That should be adjustable.
• Display a link encouraging people to sign up
  • TODO! Make the link go somewhere.
  • TODO! Display the cost to install - with suitable discounts for bulk buy.
  • TODO! Draw some pretty graphs
• Uses Picnic CSS to make it look pretty.

It is a rough and ready demo, but it does work!

The code is freely available on GitHub if you want to build something more useful with it.

We have 5kWp of solar panels on our roof.

The panels generate about 4,200kWh per year. Mostly in summer, but a decent amount in winter.

Over a year, we export about 2,800kWh. That is what we sell back to the grid.

We get paid an average of about £0.22 for every kWh we export. That's about £600 per year in tax free income.

We use about 1,400 kWh directly from our panels. When the sun is shining, we don't buy electricity. Assuming electricity prices of £0.30 per kWh, that's a saving of £400.

We earn £600 and save £400. That's a total yearly "income" of £1,000.

The average installation of domestic solar panels is - very roughly - £5,000 to £8,000. You might find a group-buy scheme which does it for less, or you may have a really awkward roof and find it costs much more.

So, at the current prices solar panels will pay for themselves in ~5-8 years.

If electricity prices rise, and export prices also rise, that timescale will drop.

If electricity prices fall, it will take longer to pay back.

If the price of solar installation continues to drop, it becomes a no-brainer to get solar installed.

Given another 20 years of generation, that could turn a healthy return on investment.

There are no on-going maintenance costs with solar panels. The panels are usually guaranteed for 25 years. It is possible that the inverter needs replacing (usually guaranteed for 10 years. Cost of a few hundred). And, like any electrical installation, things can break.

I should note that we have 2kWh of batteries. That will affect the ratio of how much electricity we use and export. It also adds to the cost of installation.

We use Octopus Energy for Solar Export (join and we both get £50). Their "Agile Export" pays a variable amount depending on electricity demand. It is worth shopping around to see if anyone offers a higher payment for export, or a lower price for import.

Solar has high upfront capital costs. Not everyone has £6,000 laying around. But if you can afford panels, they can dramatically reduce the amount of money you pay for electricity.

The good news is that solar panels work when it is sunny (duh!) - but the bad news is that electronics are generally less efficient when it is hot.

Here are a few examples.

July 16th was a few days before the heatwave:

Edent's Solar Panels

@Edent_Solar

Today I generated 25.19kWh of solar electricity. pic.x.com/mhx3ca7gyz

---

A beautiful summer's day. Not a cloud in the sky. And it generated over 25kWh of electricity.

So how did the soaring temperatures change things? July 19th was the hottest day on record. Here's how the panels performed:

Edent's Solar Panels

@Edent_Solar

Today I generated 22.71kWh of solar electricity. pic.x.com/dvvxskkxdl

---

Not bad at all! Around 12% lower on the hotter day than the cooler day. You can see from the graph that the maximum generation at solar noon is lower on the hotter day.

I don't regularly monitor the weather - but perhaps I should! It is on cooler days that our panels can hit their maximum generation. In May, because it is cooler, the photovoltaic conversion is more efficient:

Edent's Solar Panels

@Edent_Solar

Today I generated 20.55kWh of solar electricity. pic.x.com/va7k6ewoyu

---

As you can see, the panels clip off just under 4kWp. That above example has a lot of cloud cover. Let's take a look at July a few years ago. A a few clouds in the sky, with the temperature hitting about 19°C:

Edent's Solar Panels

@Edent_Solar

Today I generated 28.37kWh of solar electricity. pic.x.com/yvuupc1umt

---

Significantly better energy generation!

So, based on all my data, it's apparent that solar panels continue to work well in the new extreme heat that we have. They work better when it is colder.

Well, OK, not in the extreme cold!

Terence Eden is on Mastodon

@edent

Solar panels? More like SNOWLAR PANELS!
😂
☀🚫🌨 🏡 pic.x.com/biy8nqcfdn

---

Photograph © Skylar MacDonald 2022. All Rights Reserved. The moral rights of the author have been asserted.

The universe, of course, had other plans. Striking down both Liz and me with COVID literally the day before we were due to pick up our camper. FUCK YOU UNIVERSE. FUCK YOU COVID!

Thankfully, our symptoms weren't too bad (thanks to some TOP SECRET medical experiments we may or may not be participating in. Shhhh!)

And, even more thankfully, Jonty and his crew of joyful nerds found a way for me to do my presentation. There's this brand new technology called "Zoom". I really think it could be even bigger than ICQ messenger because - get this! - you can send voice and video and slides at the same time. MAGIC!

So I hastily rejigged my slides, gulped down some industrial-strength cough medicine, and said hello to all my new friends.

I asked the tech team to mount my laptop so I could see the smiling faces of anyone who'd entered the tent. It almost worked.

It's always hard presenting to a room where you can't hear people chuckling at your jokes. It's worse when you can't even tell if they're humans or poorly constructed animatronics. But I got a little bit of audience participation going and, overall, it went pretty well, I think:

The wonders of technology! Despite catching Covid, @edent was still able to give his talk about solar panels #EMFCamp

Dan Pope

Dale Lane (inactive)

@dalelane

such a shame I didn’t get to see @edent in person, but even over video call his #emfcamp talk was a fun, geeky dive into solar power at home
emfcamp.org/schedule/2022/… pic.x.com/2au0elao5k

---

Really enjoying @edent's Solar Punks Assemble talk at #emfcamp2022 - lots of knowledge and lovely graphs!

Aidan Taylor

John Hegarty

@johnhegs

Replying to @edent@edent @klkl @emffilmfest Really enjoyed the talk. Gonna get some quotes in for solar panels now !

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Loving @edent’s enthusiasm for solar @emfcamp - great spreading of the word!

Steve Pike

Harvinder Atwal (totally not on mastadon)

@harvy2004

After a late night (3:30am at the arcades) #emfcamp today is a slow start. First talk I went to was cancelled but got an impromptu talk on ADHD instead. Now onto solar panel data.

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Harvinder Atwal (totally not on mastadon)

@harvy2004

Replying to @harvy2004pic.x.com/j94u87xacj

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ada

@Ada Rose Cannon

We thought your EMF talk was really good. Our building collective is thinking about putting solar panels on the roof of here.

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Sadly, it looks like official videos might not appear this year:

Electromagnetic Field

@emfcamp

Talk recordings will be delayed this year, and it's possible some talks may not have been recorded.

We'll provide updates as soon as we know more.

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But... Because I love you all very much, I did a pre-record just in case the live-stream failed. My brain and throat were slightly addled with Covid when I did the record - so I ask you to bare that in mind. You can watch it for free at:

If you'd like me to natter on about solar in person - buy me a pint first ☺

What I'm saying is - get a couple of local installers to give you quotes. The technology is improving all the time - but the cost of labour is variable. Expect to pay somewhere between £2,000 and £12,000 depending on how many panels you need and the cost of all the associated extras.

The next question people ask is "how much do you save?" The answer to that is slightly easier to calculate - but highly variable!

On a typical Spring week, the panels generate about 66kWh - with some days generating nearly 18kWh!

Edent's Solar Panels

@Edent_Solar
Automated

Today I generated 17.65kWh of solar electricity. pic.x.com/Y61AR4B5F2

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We used about 31kWh of electricity directly from the panels that week. At our electricity prices, that's a saving of £6.37.

We also sold the excess 35kWh for £6.23. So that's about £12.60 in total for a bright and clear Spring week.

Across the entire year… Well, it's complicated. Some days are sunny, some aren't. Sometimes we're working from home and consuming electricity in the daytime, sometimes we aren't. The price at which we sell electricity back to the grid is highly variable. Electricity costs have been rising, but maybe they'll go back down again.

Very roughly - and this is going to depend a lot on your local costs and local usage: our 5kW array of solar panels will save us about £800 per year!

That's a saving of £300 from using electricity straight from the panels, and revenue of £500 selling electricity back to the grid.

If electricity prices keep rising, the amount of money we save will also rise.

We're with Octopus Energy - join and we both get £50. They do a specific export tariff which pays a variable price depending on the time of day you sell your electricity. They also have tariffs if you're planning on charging an electric car overnight.

What I'm trying to say is this. Solar isn't the future. It is now!