The major drawback of including electricity in the rent is if/when your tenants use more electricity than you would've ever guessed. I tracked w/d use for my tenants one month. They -- a couple -- used the washer 40 times over a 2 week period.
There are 2 meters, right? So you'd need 2 different systems?
Personally, I'd install the panels, increase rent by $100, advertise that it's all solar, and let them cover the electricity, if any.