Thanks to the great work by @peter I now have automated control over my powerwall. This lets me top up my battery based on tomorrow’s weather forecast. I currently do this by an IFTTT channel that uses weather underground to turn on a switch if it is forecast to be sunny the next day.
This does have its downsides however, as yesterday was forecast to be windy and rainy, but there was intermittent sunny patches, which actually generated more power than the battery had space (so I didn’t need to top up the battery).
So the question here is: What do people use for more accurate solar forecasting? anyone seen a decent (and free) API that can work with Home Assistant?