Technology

they generate about 3,800kWh per year. We also use about 3,800kWh of electricity each year

Obviously, we can't use all the power produced over summer and we need to buy power in winter. So here's my question: How big a battery would we need in order to be completely self-sufficient?

O, god, it's going to be huge. You really can't do the off-grid thing unless you have enough power production to satiate you over any given 3-day moving window. Trying to store power from summer until winter is going to be too expensive, instead buy more panel.

This isn't even going into the fact batteries lose charge slowly. So any power generated in summer will be much diminished by winter, even if you have big enough batteries.

What I want to do is find out what the maximum size battery I would need in order to store all of summer's electricity for use in winter.

I mean, I think that it's probably not a good idea for this guy to try to go fully off-grid if he has access to the grid, but for the sake of discussion, if one were honestly wanting to try it and one is in the UK, I'd think that one is probably rather better off adding a wind turbine, since some of the time that the sun isn't shining, the wind is blowing.

https://www.statista.com/statistics/322789/quarterly-wind-speed-average-in-the-united-kingdom-uk/

Wind speed averages in the United Kingdom are generally highest in the first and fourth quarters of each calendar year – the winter months.

The UK is one of the worst places in the world in terms of solar potential:

https://globalsolaratlas.info/

But it's one of the best in terms of wind potential:

https://globalwindatlas.info/

Basically why the grid exists to begin with. You're not supposed to be solving these engineering problems on a household budget inside a single home.

You'd be better off simply reducing your consumption or finding alternative methods of power (nat gas or maybe wind or geothermal) during the longer winter nights.

If you really want to go crazy, you should consider investing in a bigger home with better insulation and roommates. An apartment/condo block can at least leverage economies of scale, if you're dead set on DIY. More people benefiting from the setup dilutes the cost per person.