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.

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.

The EBL C9010N has a USB micro socket. *sigh* We live in the future now. Don't buy anything which requires you to have multiple cables and adapters. In terms of what the products does, it is… basically fine. Plug in Ni-MH or Ni-CD batteries and they will slowly charge.

The batteries go in at a slight angle, which makes them slightly easier to get out.

There's an LED to show you if individual batteries are charging (red) or full (green).

And… that's it! There's no WiFi, Bluetooth, or percentage indicator - just individual LEDs. It won't ping you to say it is charged. But then, it is only a tenner.

Build quality is adequate. It is light and doesn't get too warm when charging. It clicks very gently as it charges. There's a misspelling on the back, indicating that it takes Ni-NH batteries.

If you want a cheap way to recharge batteries, this is fine. It's about a tenner for the charger by itself. If you want a charger and 8 batteries, that's about £16.

That's reasonable value for money. But, personally, I think you should treat yourself to something with more features and a modern USB port.

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.

So let's cut out the middle-man and plug a USB-C cable straight into our batteries!

What?

These were the cheapest AA batteries I could find which took USB-C. £16 including delivery, for 4 batteries and a mutant cable. Regular AA batteries are about a quid each, or 50p if you buy in bulk. So these only have to last 8 times as long as a normal battery to be worth it.

The mutant cable causes fear and despair to all those who look upon it.

The triple-ended USB cable is for power only. It happily charged my phone but didn't let any data through.

Plugging the batteries into the cable set of this flashing green LED. The light goes solid once fully charged.

I'm half disappointed that they don't come with BLE and an app to say when they need recharging!

A quick shove into a battery tester showed them to be pumping out about 1.45V.

The other (non-rechargable) batteries I have give out 1.3V - 1.6V, so these seem perfectly acceptable.

The USB-C batteries are identical in size to their standard brethren.

Do they really have 12800mAh capacity? Duracell's AA batteries claim a maximum of 2500mAh - and they don't need space for a USB-C port and LED. So, I think that capacity is unlikely. It wouldn't be the first time Daweikala have misrepresented their batteries' capacity.

I don't have the tools (or the patience) to properly evaluate their capacity. All I know is... They work. I shoved them in my gadgets and they provided enough power. If they drain a bit quickly, I'll shove in one of the hundreds of USB-C cables I've got laying about the place. The batteries get slightly warm while charging, but they don't emit any noise or magic smoke.

So, they probably lie about their capacity and they're a good deal more expensive than regular batteries. But they're quick and easy to recharge, and they create less waste.

You can probably find them on AliExpress, or in your local equivalent, at a cheaper price.

One day, in the glorious future, every device will charge via USB-C. Until then, these are a nifty way to retrofit old gadgets.

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.

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.

My friend Paul Curry recommended the APC SMT1500i. It usually retails for around £700, but I found a refurbished one for £200

Let's put it through its paces!

It's a chunky monkey! And weighs about 25Kg. So best set it on the floor, or a really sturdy shelf. It's about the same size and shape as a PC tower - with an array of blinkenlights and buttons on the front.

It's pretty simple to use. Plug it in to the mains using the supplied kettle leads. It'll make some beeping noises and the batteries will start to charge. This causes the fans to spin. They're not overly noisy - but they turn off after 3 hours when the unit is fully charged. The unit sits passively quiet until there's a power cut.

You'll notice that on the back are eight IEC sockets. You'll need an IEC to UK plug adapter in order to connect your electronics.

And... that's it. I charged the UPS, plugged my kit in, then unplugged the UPS from the wall to simulate a power cut. It immediately started beeping, to let me know it had lost power, and its little lights started flashing. And, most importantly, all of my gadgets stayed on!

It can output 1kW for about 6 minutes - for a more modest 400W, it'll run for half an hour. So should be able to keep most gadgets going but it isn't going to run your gaming rig for several hours.

Importantly, it has serial control, so you can tell your servers to gracefully shutdown when the power goes out. Let's take a look!

Linux Usage

The RJ45 port is not, sadly, Ethernet. It's a serial port. Kinda redundant as there's a USB-B port as well. Plugging it in to a Linux machine gets you this on lsusb: 051d:0003

Although APC provide their own software, it seems a bit outdated and doesn't fully support Linux.

There's a good tutorial to get the UPS talking to Ubuntu.

Once that's done, you can monitor the battery from the command line. I wasn't able to find a way to upgrade the firmware, or do anything fancy like that. But it seems to show all the basics.

Overall, it's a great little UPS.

It's pretty similar to most normal portable batteries - but because it has USB-C-PD, it can charge MacBooks, Nintendo Switches, and other high-power devices! Let's take a look:

In the box

A big black battery.

A weird hybrid cable.

It is USB-A to Micro-USB and has adapters for USB-C and Apple's Lightning. It isn't a USB-C PD cable.

There's also a carrying case, but the battery feels rugged enough to shove in pocket or backpack without any further protection.

Spec

The batteries hold roughly 100Wh of power. That's about enough to charge an iPhone 6 times. Or a MacBook Air twice. Because of its huge capacity, it takes roughly 4 hours to fill from empty.

It has a lot of ports!

The USB-C port is bi-directional. That means it supports Power Delivery in both directions. It will charge the battery, or power a device.

There's also a micro-USB socket if you want to charge it the old-fashioned way.

If you don't have USB-C gadgets, there are a pair of regular USB sockets. Both can be used at the same time for charging multiple gadgets.

Finally, there's a button to press which lights up some LEDs. That lets you see how much power remains. Handy!

For spec nerds (like me) here are the power levels.

Testing

This battery can power THREE devices at once! I had the USB-C slot charging my laptop, the USB-A slot charging my phone, and the other USB slot powering my headphones.

Both my Linux laptop and Chromebook charged via USB-C PowerDelivery. I didn't do a full load test, but it appeared to be giving out 30 Watts.

It had no troubles powering the Nintendo Switch.

It happily recharged all my USB-C gadgets, and my legacy USB things.

The unit charges via USB-C as well. I left it for a few hours and it was completely full. There's also a micro-USB socket if you want to charge it slowly. The LEDs blink upwards as it charges.

Because of its large capacity, you should check whether you can take it on an aeroplane. Current rules suggest it is suitable for carry-on luggage.

Verdict

For £25 on Amazon you get a lot of battery. It is big and heavy - but that's a limitation of the technology. There's no PD cable included, but you probably already have one if you have a lot of gadgets.

The USB-C Power Delivery is a game-changer. You can take this battery with you and be sure it will charge even the most demanding of devices.

Amazon Alexa is a fun little bit of kit. But it can be tricky getting it to work with all your smart devices. Not every company has an Alexa skill - just like not every company has an app.

Using Flask-Ask it is possible to bring Alexa smarts to a range of previously mute devices.

Alexa coding works on "intents" - the following is a simple intent. That is, you can only ask the skill one thing. No state is maintained, no multiple commands to get right, no complexity. This gets information from a single source and speaks it.

Code

The basic Python is pretty simple and can be adapted to query almost any basic JSON API. Let me walk you through it.

All the boilerplate needed to set up the skill:

import logging
from operator import itemgetter

import requests
from flask import Flask
from flask_ask import Ask, statement

app = Flask(__name__)
ask = Ask(app, '/')
logger = logging.getLogger()

@ask.launch
def launch():
    return stats()

The API that you want to call. This is a basic JSON API which doesn't require authentication.

ENDPOINT = "https://example.com/api/maslow.json"

Here's the main part of the Skill. When the intent is triggered, call the API. Get the data and format it for speech. If there was an error, tell the user.

Make sure that the name of the intent is identical to the one you set up in the Alexa Developer console.

@ask.intent("BatteryIntent")
def stats():
    r = requests.get(ENDPOINT)
    data = r.json()

    if r.status_code == 200:
        percent = data['battery/amphours']
        speech = "Your Moixa battery is at " + str(int(round(percent))) + " percent capacity right now."
    else:
        speech = "There was a problem connecting to the battery."

    logger.info('speech = {}'.format(speech))
    return statement(speech)

Short and easy.

Deploying

Again, Amazon don't make it easy to deploy Alexa skills - here's a guide to getting started using Zappa.

Remember, Alexa is not AI. You must painstakingly type in all the "utterances" that you think your users might say to activate the skill.

Problems

There is one major problem with retrofitting Alexa skills.

Lots of Internet connected devices have no ability to log in remotely - and certainly don't have the OAuth systems that Amazon demands. Alexa has no ability to directly connect to IP addresses on its own subnets.

This means most skills will need hard-coded credentials - and a way to traverse into your network.

In short, this means means they can't be shared on the Amazon store.

Now, sadly, the new battery has failed. The phone will suddenly switch off as though the battery were yanked out. After a few minutes it will happily turn on again. Most frustrating.

Rather than go to the expense of replacing the entire handset, or trying my luck with a different battery, I decided to try a £18 extended battery case.

The case is large and adds a bit of heft to an already plus-sized phablet. It has a fairly grippy rubberised plastic coating which makes it pleasant to hold, and unlikely to slip away from you. There's a single button which is used to activate the charger.

Once the phone is clipped in, it begins to charge immediately. The four LEDs at the bottom give you an indication of how full the battery is. The LEDs are ridiculously bright - it's unpleasant using the case in a dark environment. A small bit of black tape sorts out the problem.

The cut outs are well placed and don't interfere with the headphone jack. Similarly the power and volume buttons are easy to access. As you can see, the phone doesn't quite sit flush into the case. There really ought to be a way to tighten down the top.

It's a chunky beast - but the 4,200 mAh battery demands that size.

There's a recessed kick stand which, while feeling slightly flimsy, holds up the phablet at the perfect angle for watching movies.

The case doesn't quite snap shut fully - and there's no locking mechanism. This means the top is prone to shifting open.

The camera is quite recessed - but it doesn't seem to reduce the photo quality.

Overall, it works well. Depending on how you use the phone, this will dramatically increase your "screen-on-time". If you have the pocket space, I recommend it.

Downsides

It's good - and a great price - but it's not perfect.

• The maximum power output is 0.8 Amps. This isn't a fast charger, but it's good enough to trickle power into your battery throughout the day.
• Because Android sees it as a charger, your phone may not go into deep sleep.
• It charges itself at around 1 Amp - so expect it to take a few hours to fill up from empty.
• Like all cheap chargers, it buzzes a little when in use. Sounds like an old fashioned hard-drive chittering away!
• There is a design flaw on the N6 which makes the touch-screen slightly erratic when charging. It also produces a faint buzz in attached headphones. This occurs whether using a stock charger or this battery pack.
• The case is a little loose at the top. It doesn't feel like it's going to pop off and release the phone, but it's not reassuringly snug.
• Blue LEDs are too bright.
• No Qi built in. The case is too thick to let the phone's built-in wireless charger to pick up the power.
• Upside-down USB port. Can't really fault the manufacturer for this - but Motorola's bone-headed decision to invert the port continues to annoy.
• Even for someone with fairly large hands, the size and weight takes getting used to.

If you've got a dodgy battery and don't want to disassemble the phone - or if you want to get through a whole day of use - I can recommend the £18 Moonmini extended battery case.

Thoughts on Mobile Battery Life

4 years ago, I tried a similar battery case for the original Samsung Galaxy.

It strikes me that not much has changed in the world of Android. I struggle to actually use the phone for more than a few hours each day before the battery dies. Fat battery packs double the life at the "expense" of portability.

Don't get me wrong, Android is becoming more efficient - it can push more pixels and transfer more data on battery packs which are barely bigger that when it was first released. The HTC Magic was released 6 years ago and had a 1,340mAh battery - the latest Nexus 6P has a 3,450mAh battery.

The state of the art is 2.5 times the battery power, but still barely a day of real use.

While Moore's Law still holds, we can expect a doubling of processing power ever 18 months - sadly battery technology just hasn't kept pace. While Marshmallow's "doze mode" helps, perhaps it's time for Android to concentrate on efficiency rather than "beautiful" animations?

Taking apart the nook is incredibly simple, pop off the power button, use a Torx 5 screwdriver to undo the single screw, and then spludger your way around the edges. This is what I found in mine...

VERY BAD!

A replacement nook battery is less than a tenner from Amazon. It is developed by Cameron Sino who seem to specialise in making replacement batteries for all sorts of gadgets.

Fitting the battery was a little tricky, here's how it went...

There's a small port on the motherboard into which the battery cable is attached.

The cable only fits in one way - with the red wire at the bottom. This is the position which the battery will take when the nook is reassembled.

The battery came charged, so I was able to verify that everything worked as expected. Now's a good time to test charging it.

Once done, unplug the battery and prepare to reassemble!

The back case of the nook has a recessed area for the battery. There's just one tiny problem...

Yup! The battery doesn't fit!

Luckily, the casing for the battery is rather pliable and, with some gentle force, it's just about possible to squeeze it in.

A little bit of tape to secure it into place and we're good to go!

The cable on the battery is long enough to plug in while the back is detached.

Reassembling is somewhat tricky. You have to slide on the bottom, then re-clip everything, and slide up. Be careful not to apply too much pressure - it's really easy to damage the screen that way.

When all the plastic casings are done, it's a simple case of screwing in the T5.

And - that's it! A fairly easy device to replace parts on - no soldering, easy clips, one screw. Why can't all electronics be like this?

Replacement batteries for the nook and glolight are on Amazon for under £10.

Eventually, the battery failed. It would report being fully charged, but only run for a few minutes before switching off. Recalibrating and resetting the battery statistics failed to improve the situation.

So, time to buy a replacement Nexus 6 battery. Cost under £15 and delivered on the slow boat from China.

I followed this excellent tutorial from iFixit. Here are my notes.

• Buy proper tools. You'll need a T3 Torx head. That's quite an uncommon size, so I picked up this torx set which also contains a set of tweezers.
• Melt the glue gently. Most of the glue had already perished on mine, but for the elusive splodges you'll want to gently apply heat. I used a hot water bottle filled with boiling water. I rested the phone on top of it for a few minutes and then gently peeled it apart.
• These are delicate electronics. Be very careful. Don't tug.
• Once you've undone all 22 T3 screws, the phablet opens into two pieces.
  
• As you can see, the old battery was wrecked
  
  Although, to be fair, some of that disfigurement is caused by prying off the glue. Remember to recycle your old battery safely.
• When replacing the battery, you have to consider whether you want to glue things down again. If you don't, there's a slight risk that some of the precise connections could shift and you'll have to disassemble everything to realign them. I cheated an used a small piece of sticky tape to keep the Qi in place.
• You don't need to use the Qi inductive coil - although it also contains the NFC antenna as well. If you break it, you can buy a replacement fairly cheaply. The quad connector which wraps around the battery is for Qi charging. The long straight cable is for the NFC.
• I didn't screw everything back together until I was sure it had worked. I put in the four screws for stability and then tested both the USB and Qi charging. I also tested the NFC using a basic app.
• Success!
  
• The battery came partly charged - I reset the battery statistics just to make sure it was working. Once fully charged, I'm going to use it until it is completely empty.

There you have it! For around £25 including tools, I've got a brand new battery - and a little ego boost knowing I can do basic repairs to complex electronics.

Over to Ben Smith:

... a frequent complaint is that we don’t want thinner phones. We’d rather manufacturers used space savings to provide larger batteries. You know… ones that last more than a day like they used to.

Wireless Worker

Damn straight! Do I care if my phone is a bit thicker if it means I can go all day without a charge? No. No I do not.

My Samsung Galaxy S barely gets half a day of "real" use. I tend to browse the web for an hour while commuting, listen to music over BlueTooth, make a few calls and texts, FourSquare, and take a few photos. Come lunchtime the phone is whimpering in the corner just begging to be plugged into a USB socket. So, I either have to keep my phone sucking at the teat of my laptop, or severely curtail my usage. Unacceptable.

So, I bought myself an extended battery for the SGS. Specifically, the CostMad Samsung Galaxy S i9000 3500mAh Extreme Extended High Capacity Quality Spare Replacement Backup Battery with Battery Back Rear Cover Case. Quite a mouthful, but here's how the wee beastie looks:

Total cost? £9. Less than a tenner from some random factory in the Far East.

Now, the first thing that you'll notice is that the battery is so massive that it requires a new backplate. It bulks up the phone, but does have some advantages.

It feels heftier. The weight isn't very different from the original, but it feels more solid. The slippery and flimsy backplate is replaced by a rubberised and solid case which fits snuggly.

I let the battery charge while the phone was off. Once done, I proceeded to use it as normal. At the end of the day, this was the result.

Whoa! 8 hours of, if anything, heavier than usual use and I'm still have over half of my battery power available.

So, I went home, browsed the web, watched some YouTube, played some games, and generally tried to force the battery into submission.

By this time my regular battery would have been dead and buried, but this one just kept going!

Get One Now!

I could never buy a phone with a non-removable battery unless I was sure that it was of sufficient capacity to keep me going throughout the day. Sure, you can buy portable chargers, solar chargers, even hand cranked chargers - but that's just a stop-gap as far as I am concerned.

I could keep my phone plugged in - but that rather defeats the purpose of a "mobile" phone. As I head off to Mobile World Congress, a battery like this is a must.

Some Notes On Testing

• I'm running a beta build of ICS. That may not be particularly well optimised.
• I keep my screen brightness on lowest rather than auto.
• I'm usually in WiFi coverage.
• I have push notifications for Gmail and Exchange set on.