In by far the most exhaustive and detailed study to date, the National Renewable Energy Laboratory (NREL) found that solar homes sold 20% faster, for 17% more than the equivalent non-solar homes, across several subdivisions built by different California builders....

After extensive interviews with the home buyers in the development, the (413 page pdf) NREL study made some other interesting findings.

If solar was already on the house, and factored into the price already, buyers were more likely to pick a house with solar. But if it was just one more decision to be made at the point of purchase, the decision got shelved. (more)