I'm working on a solar forecast for my off-grid PV installation. It's a small install of 1kWp (2 bifacial panels) with a 4kWh battery. I chew through about 25% of my battery each day and so a forecast of the next few days or so is very useful to help me decide whether I can dip heavily into the battery or whether I should be a bit more conservative about power use.

https://tim-burgess.observablehq.cloud/pvoutput/solar

SolCast (solcast.com.au) has a free API for home users and also some historical data. So it's essentially just some D3 code running on Observable.