30 months to 3MWh - some more home battery stats

Back in August 2023, we installed a Moixa 4.8kWh Solar Battery to pair with our solar panels. For the last year and a half it has chugged away slurping up electrons and sending them back as needed. Its little fan whirrs and the lights on its Ethernet port flicker happily as it does its duty.

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.