In addition to Will's channel, don't overlook thermal battery links, too, which often can be made out of old water heaters that folks dump on craigslist all the time. The gist behind thermal batteries is to help with the (solar->battery->coil->hot water) efficiency loop by changing it to (solar->coil->sand->hot water) e.g. https://hackaday.com/2022/11/21/making-a-do-it-yourself-sand... (I love his channel, too) or https://www.technologyreview.com/2024/04/18/1091481/how-to-b...

They also have vastly different failure modes than pumping a bunch of DC voltage into battery cells

There was also recently a post about flow batteries <https://news.ycombinator.com/item?id=41235789> which are also very interesting to me, but I haven't gotten far enough into that learning curve to know if they're a good fit