I built this to plan the energy flow of our DIY solar heating system. We had spare LiFePO4 batteries, a second-hand inverter, 8 panels and some MPPT trackers — so instead of buying something, we wired a 1.3 kW heating rod into an 800L buffer tank. The calculator has three tabs: daily charge estimation, summer discharge/hot water usage, and transition season planning. Single HTML file, vanilla JS, MIT licensed. Might be useful if you're doing something similar.

More info about the solar project https://achildrenmile.github.io/posts/pufferspeicherrechner