A letter in the current Harvard Magazine, by an astronomer at UCLA, prompts me to blog about my photovoltaics. Professor Jura has described his experience here.
I live in an old house in Brookline, perhaps half a mile from the Brigham and Women's hospital. One day I noticed that, in spite of the houses around us, the flat roof on the garage has a pretty clear southern exposure. After getting in a contractor and running some financial models, I determined that a PV installation would pay for itself in about 7 years. It went online in May, and so far the energy production is a little ahead of the projections. Generates 5 kW peak and should produce about 7 MWh/year. The financial model has three components: tax credits for part of the capital investment; reduced electric bills because I pay only for net usage; and income from sale of State Renewable Energy Credits, one per MWh I generate independent of my electric usage.
System is depicted below. It is all but invisible from the street. And you can look here to see its production, either power or energy, by either day, week, or month. PV panel prices have softened in the past year so I'll bet it would have a shorter payoff period if I were starting today. Glad to answer any questions offline.