Solcast offers a free API toolkit that is pretty good for solar forecasting. Down the line (and unfortunately this is still a way off) we are keen to integrate this function into our smart meter service and in turn the HEMS - unfortunately no dates for that at the moment, so in the meantime if you are comfortable working with the API Solcast is definitely worth a look!
Make sure to keep us updated with how you get on, I would be interested to hear about your experiences of the accuracy and functionality that you are finding with this!