Assinging a number to that amenitiy in this market is really meaningless. There are too many other factors that are influencing this market to assign a premium to something that few buyers assign a priority to. To illustrate lets say your home doesnt have good flow, a poor kitchen and no updates but it has solar power. Not too many people are going to jump on that especially if the home next door has good flow, kitchen and upbates with no solar.
In a "normal" market, one might be able to quanitfy a number this way. If the power bill goes down a significant amout to enable a buyer to by a larger home (Since his monthly housing cost would drop and that money could be used to make a higher payment). The cost offset would affect a buyers debt to income favorably (more income dropping that ratio). Unfortunately many bank underwriters do not consider this criteria seriously.