One MegaWattHour of Battery Power!


Less than a year ago, in August 2023, we installed a 4.8kWh Solar Battery at a cost of £2,900. Whenever I talk about the upfront capital costs of solar power, people rightly want to know what the payback period is.

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.

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


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11 thoughts on “One MegaWattHour of Battery Power!”

  1. @blog I remember seeing an article years and years ago about long running computations, saying that if your computation was going to take more than $x years to complete, and you assumed that processors would get faster every year by $y%, then it sometimes made sense NOT to start straight away. If you waited until processors were faster, you could start later and be finished sooner.

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    1. @blog So with your battery, the figure I'd want to work out is not "how long does the battery take to pay for itself" but "If we wait and buy a battery next year - that we can assume will be better and cheaper - will that battery finish paying for itself BEFORE one that I bought this year?"

      My gut feeling right now is that it would.

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  2. @blog I'm anticipating going down the solar panels + battery route at some point in the next 6-18 months, and one of my concerns is that a lot of these systems advertise an app for monitoring/management --- and that seems like something that might go unmaintained relatively swiftly, specially if hosted cloud facilities are involved.

    Do you happen to know whether your system, at least, can operate independent of any cloud service and provides a general-purpose API?

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    1. @dwm @blog
      My system is, sadly, cloud tied. That was a significant concern for me but sadly couldn't be avoided.
      It will continue to work when disconnected from the Internet - albeit not as efficiently.
      I have some confidence that the company will open a local API if necessary. But, at the time, I couldn't find any purely local solutions that I was happy with.

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  3. said on irrelephant.co:

    @Edent I am considering doing it when we redo our roof (don’t want to install anything on an old roof) but my main problem isn’t the tech or even install cost, it’s how scammy the whole commercial aspect around it is (at least where I am). All I see is over inflated claims, financing offers, etc, when all I want is a quality setup with reliable support if needed…

    Reply | Reply to original comment on irrelephant.co
  4. @blog #homeassistant has quite a lot of 'integrations' for several manufacturers. Maybe that could help in the long run? Also some apps/manufacturers for solar equipment only allow you to access broad long term statistics and a few current days of detailed info if you don't pay for premium access. Home assistant can save and access data for the whole measured period "for free" (if you don't count setup, hardware and electricity).

    Definitely something I will consider if/when I get into solar.

    | Reply to original comment on det.social

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