The payback period for solar is a function of electricity prices. For example here are some commercial and industrial kWh pricing that Solar CFO uses to underwrite our projects:
Hawaii has some of the highest electricity prices in the country at about $0.30 per kWh.
California also has high rates: SDGE approx $0.21 per kWh, PGE approx $0.18 per kWh, and SCE approx $0.14 per kWh. There is also a great program within the LADWP territory which allows owners to max out their rooftops with solar and sell into the grid for $0.135 to $0.145 per kWh.
Massachusetts has a state program that buys electricity for approximately $0.16 to $0.24 per kWh for 20 years (based upon a sliding scale and your region)