As a local expert on solar homes I was pleased to see the Appraisal Institute announced their conclusion on the topic of solar's impact on property valuation: 

Solar Electric Systems Positively Impact Home Values

To determine the actual number for the value impact is not an easy task. And, it has very little connection with what the homeowner paid for the solar system, just like anything unique thing a home owner does to their home.

But at least appraisers now have a tool to use to get a handle on the impact of a solar system on a home's value.

The Appraisal Institute worked with scientists at Sandia Laboratories to pin down some data that an appraiser can collect from the homeowner or the installer of a system. Inserting these numbers in to an online spreadsheet can pop out a dollar amount that the system might contribute to the home's value.

The tool is usable for both photovoltaic (PV) solar, (Which generates electrical power) and solar thermal water heating, which is typically used for domestic hot water, but can be used for domestic space heating in extreme installations.

Now what the market impact of a solar array is will take a while to determine, but the work of Sandia Laboratories and the Appraisal Institute is a great start!

Here is a link to the Institute's press release: http://www.appraisalinstitute.org/newsadvocacy/news/2013/Solar-Electric-Systems-Positively-Impact-Home-Values-10-31-13.aspx

Here is a direct link to Sandia's Information: http://energy.sandia.gov/?page_id=8047

And finally, here is a link to the form for collecting the basic solar system data:

http://www.appraisalinstitute.org/education/downloads/ai-reports/AI-82004-res-green-energy-eff-addendum.pdf (Note the link in the press release to this form doesn't work. This link does as of 20131210.)