It seems to me that using a metric that is as close as possible to the actual desired outcome is the way to minimize the potential of gaming the system (i.e. Goodhart's law). At the end of the day, the desired outcome is more closely related to net energy consumption (per building, square foot, person, or similar) than to the number of solar panels on roofs. Of course, this is assuming that the regulations come out of sincere concern for the environment and/or energy grid, not concern for special interests like solar panel manufacturers or construction companies.