This is probably becoming more, and more of a factor as homeowners realize a savings on their energy bills:
However, at this time; it is like asking how much value a GAS range adds to the house, or a remodeled Kitchen:
Some buyer will perceive the added vale, and others will not.
If the house needs a lot of repair and has poor curb appeal; the Solar Panels will not sway anyone.
If the house has been refurbished, upgraded Kitchen and baths, hardwood floors, fantastic curb appeal, AND solar power; then it will command a premium price.