Solar Graphing FAQ

You're probably here because you saw an auto-tweet from my solar panels. Yes, that's right, my house is wired up to the Internet of Things!

Here are answers to some questions I get asked about them. If you've got any others, please stick them in the comments section or ping me a message.

How much did the panels cost?

About £8,000 from Sims Solar. That included panels, inverter, data capture card, installation, and an iBoost. Full details on the installation are available.

What's the payback rate?

We're expecting the panels to pay for themselves in about 10 years. The government gives us tax free cash for every kWh we generate. Our electricity bills are lower. Any excess energy goes into our immersion heater to heat up water. The rest is sent back to the grid.

How much power do they generate?

It varies. According to this solar calculator we should generate around 3,400kWh per year. Obviously, that will vary throughout the year.

How much electricity do you use every day?

Based on our last four months (September-December), we use an average of 8.5kWh per day. There are only two of us in the house, but we have a lot of computers running. We've switched to eco-lightbulbs, and have high efficiency kitchen appliances.
According to the UK Government, the average household electricity usage is 3,300kWh per year. That's ~9kWh per day.

So, theoretically, we could generate enough solar power to offset our entire energy consumption.

Is British weather sunny enough to make this viable?

Yes and no!

Even in winter, close to the solstice, I was still able to generate ~10kWh on a clear day.

A few days later, however, the rain and fog resulted in this pathetic specimen.

Can you go off grid?

Not really. If there's a power cut, the inverter stops feeding energy back into the grid lest it electrocute someone.
That said, all my battery powered gizmos charge during the day - so they'll be ready come the zombie uprising.
I am thinking of adding a UPS to keep my computers running in case of power failure.

How did you hook the panels up to Twitter?

I wrote a script to run on my server, monitoring the panels. At sunset, the script draws the graph and tweets it. All the monitoring code is Open Source on GitHub if you want to play with it.

What does "Feeding Into The Grid" mean?

During the day, if I'm out, I still have electrical appliances running - computers, security cameras, WiFi, fridge, etc. Let's say that the total draw of those is 500W.

If my panels are generating 600W, the electricity used by my appliances comes directly from the sun and not my electricity company.

The excess energy (600W - 500W = 100W) flows back through my meter and is used by everyone else on my street. It also has this side effect:

But solar panels don't work during the night!

An excellent observation. Luckily, I get to work from home a far bit. I also schedule my washing machine and tumble dryer to come on around mid-day. Anything I don't use goes into my hot-water supply. Anything left over after that goes to giving people on my street clean, green energy.

21 thoughts on “Solar Graphing FAQ

        1. Yes, that's right. The panels practically cover the entire roof. They're facing a few degrees west of South. As for angle we're limited by the slope of our roof.

  1. is wind an option in your locale? It would serve me well on my mountain top location but the sum of wind and solar together would fare better in covering each other's outages if integrated via a large enough battery bank. Are you an EE or have training in electrical installation? If not, what books did you read to come up with such an effective system?

    1. Wind isn't that great for us. We're in a built up suburban area - so wind speeds rarely get above the 5MPH needed. A small roof-mounted domestic turbine will only generate about 1kWp and cost about £2,000. A freestanding one (assuming we could get planning permission) would be about 2.5kWp and cost £15,000!
      So solar was £8,000 for 4kWp - sure it doesn't generate during the day, but it's a lot more efficient.
      I engaged a specialist to plan and install the system. Unless you're fully qualified, please don't mess with your electrical supply!

      1. I live off-grid...far less tax incentives available but I have little choice. Seems odd until you realize that in US, the legislation or regulation to create incentives for solar are lobbied for, if not written by, the power companies and the home improvement contractors. Most back up generators sold here won't work unless grid tied...the cessation of the 220V is what turns them on.

  2. Christina says:

    Did you have to pay £8000 up front? Are there government incentives for paying some of the cost of this? Thanks!

  3. Yang says:

    But you don't have the battery in this system? I think that is very important for the stableness of electricity.

    1. Terence Eden says:

      Having a battery doesn't affect the stability of the system. There's a seamless transition - the lights don't even flicker - when the house goes from solar fed to grid fed.

      1. Yang says:

        Days ago tesla release a battery product name "powerwall" , which can store 10 kwh electricity power. Do you think it is a good option for you? The battery can be charge by solar panels during the day and power your house at night. I think save a lot of electricity bill, what you think?

        1. says:

          A battery helps you keep the electricity to yourself, and might well work financially. In terms of greenness it doesn't add any electrons to the world, if you didn't have the battery then someone else would be getting your electrons and a powerstation would be working less hard. It isn't a terrible idea, and lots of them with grid signalling could add stability and smoothing, allowing the grid to have less peak power. The iBoost immersion thing is actually rather anti-social. It is a big "screw you, not sharing my electrons", all your spare power goes into making hot water whether you need it or not, rather than going to the grid. Again it might pay for itself financially, but it isn't a green thing at all.

          1. Terence Eden says:

            Hi Alan,

            Thanks for the comment. I respectfully disagree with your thoughts on an iBoost. When it is used, it means I'm not burning gas - that leads to a modest reduction in my CO2 emissions. Based on the data I've collected we've pumped ~0.96MWh into the iBoost - less than a quarter of what we've generated. Given our house's usage, we're still feeding back the majority of our generation to the grid.

            If we had a boiler which instantly produced hot water when needed, an iBoost would be unnecessary. Given the limitations of our house - and my fondness for hot showers - I think it makes sense with our energy usage.


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  5. Peter says:

    Is the data behind available in tabular format? If I got solar panels installed I'd have to split the array east/west like you, and I'd like to use your historic data to work out how much of our usage we'd cover during the winter.

    And I'd rather not scrape Twitter to get the data if I can help it!

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