Number of modules making up the installation: generator:solar:modules=*. Modern modules have a rating of between 300-370W and measure roughly 1.5 by 1 m.
EDIT: apologies, only just now noticed that new-field should also be done trough the issue creation wizard. I'll do it like that next time.
I think something useful to add to the rooftop solar preset that's in the schema; is the amount of # modules in a rooftop solar panel installation.
It's something easy that people can visually check to add.
https://wiki.openstreetmap.org/wiki/Tag:generator:source%3Dsolar
EDIT: apologies, only just now noticed that new-field should also be done trough the issue creation wizard. I'll do it like that next time.