If you use solar power in a climate like Germany you will get that kind of math. Let's say you use 8000 kwh per year, and you have the solar capacity to create

If you use solar power in a climate like Germany you will get that kind of math. Let's say you use 8000 kwh per year, and you have the solar capacity to create 8000 kwh per year. Assume you already have a normal home battery of ~14kwh so day/night is not an issue, the solar production during daytime charges the battery and all night time power comes from the battery. Now your big problem is that 70% of the 8000 kwh solar per year is produced from May to September. You need 670 kwh per month for usage, so in those four months you have just under 3000 kwh too much. Then in April and October the panels roughly produce what you use. And November to March you have a total shortfall of 3000 kwh. So you need seasonal storage capacity of about 3000 kwh to be able to run an 8000 kwh yearly usage on solar in this climate. The other option is to buy much more solar panel capacity than you need, then you can use a smaller battery which in total could be cheaper since solar panels aren't that expensive anymore.