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.

The study looked at a number of housing developments where the homes were otherwise identical except for the solar energy systems. 

Also interesting was that buyers were more interested in solar when it was-preinstalled:

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.