I just found https://solcast.com.au/api/docs/pv_power.html while looking to try to do what Tesla has been promising but have not yet delivered, but better - automate charging my powerwall overnight if the solar forecast is poor for the next day - based on the details of my system (size, orientation, etc.)
Solcast - currently free, currently in beta - gives you a forecast for a location, system size, tilt and azimuth based on their clever sauce, including things like cloud cover forecasts.
It looks very cool.
If they are keen (and, I suspect they might be, for PVOutput) for a collaboration, you could replace (or add to) your insolation forecast which is based on location tilt, azimuth and system size to one which is based on forecast weather. If they are not already using your system as a source of live data, I’m sure they’d be interested.
If I get this going, I’ll upload the expected (half hourly) output to an extended variable - it will be very interesting to see how well it tracks to actuals.