What difference has a home battery made?

Around this time last year I wrote about how we had got a battery system installed in our house. It was discounted as part of a community buying scheme, and we bought it to support our solar installation. With a year of data, I can now see what difference it has made to the amount of our own energy we can use.

The graph below shows the amount of free electricity we get from the solar panels, before and after installing a battery.

Before fitting the battery, we were getting around half our electricity from the solar and half from the grid. With the battery we’re now getting two thirds from the solar and one third from the grid.

Our usage here includes the electric car, which we charge from home. So the savings on our electricity apply to both the house and the car.

Like any solar installation this far into the Northern Hemisphere, there are big seasonal variations. Light levels drop to their lowest in December and the battery doesn’t make much difference then. From January onwards it’s making a contribution to savings, peaking in early summer where we were 98% self-powered. We’d hit those kinds of levels for more of the year if we weren’t charging the car, as the need to occasionally charge on a cloudy day is what keeps us from self-sufficiency.

As for what the long-term payoff will be and whether it’s ultimately worth it, time will tell. It partly depends on future energy prices. The price to buy electricity is so much higher than the price for selling surplus, even with the feed-in tariff, that the case for using our own power improved considerably in the last couple of years. That’s important for us as we plan to fit a heat pump at some point, and it will help to reduce the costs of all-electric heating.

I wouldn’t necessarily suggest that everyone who has solar panels needs a battery. If you have a tank, you can use the surplus to decarbonise your hot water. (We don’t have a tank in our small terraced home.) You can also treat an electric car as all the battery you’ll ever need, and put the surplus into it with a smart charger like the Zappi. But if you’re having solar fitted, it’s increasingly common for installers to offer a battery at the same time. That keeps costs down and it may be well worthwhile.