Technology

It's practical for someone with limited space for panels on a small room, but I ran these calculations by moving almost all loads to daytime, sizing the panel array to the (minimum daily usage + efficiency losses) * buffer factor for days long storms or equipment failure.

Start with the comparitively cheap panels if you have the space, move electrical loads to the daytime and design the house for thermal momentum, and size storage to the minimum inclusive efficiency losses times buffer. If you have the roof space the panels are the cheapest part and you should usually way, way over panel.

The most important thing is having thermal mass enough or living in a climate that allows your home to not need thermal input or extraction at night. Heat is expensive and exponentially moreso if you need to produce it from conventional storage.

Author's diagram is about summer. Fall, winter, spring is about heating-degree days. If you're heating your home with electricity, you'll not get there with batteries.

So, working towards a solution, there are other ways to store excess energy than in batteries. One example is sand, which can be heated to very high temperatures. Insulate a sand container well and its storage can do a lot of home-heating.

Example: https://www.livescience.com/technology/engineering/a-scalding-hot-sand-battery-is-now-heating-a-small-finnish-town

We'll need to put a lot of different methods into use. There are many practical ideas out there, and they'll need to be tried.

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/

Something very important that anti-nuclear but otherwise environmental minded people should realize is this sentence: " There's no practical way to build domestic batteries with this capacity using the technology of 2025."
Also applies to grid storage. There does not exist a chemical energy storage solution that can substitute for "baseload" power. It's purely theoretical much like fusion power. Sure maybe in 50 years, but right now IT DOESN'T EXIST. Economically, practically, or even theoretically.

Why do I bring this up? Because I've seen too many people think that solar and wind can replace all traditional power plants. But if you are anti-nuclear, you are just advocating for more fossil fuels. Every megawatt of wind or solar, has a megawatt of coal or gas behind it and thus we are increasing our greenhouse gas emission everytime we build "green" generation unless we also build Nuclear power plants. /soapbox