Conveniently, the smart folks at Lawrence Berkeley National Laboratory, along with some funding for the Office of Energy Efficiency and the U.S. Department of Energy, recently concluded a study on this exact question. Well, almost this exact question. They looked at homes in California, 72,000 to be exact, including 2000 that had solar power installed at the time of sale.
The data was spliced and diced a variety of ways; best for you to refer to the report and summary for the particulars. The study found pretty convincing evidence that houses in California, with solar systems (believe me, the Lawrence Berkeley Labs folks don't refer to them that way) sold for a hefty chunk over homes without such systems: nearly $17,000 for a newish and averaged size system.
Do Don and I think our system added that much? Not quite. California has more sun, so a similarly sized system to ours will produce more power throughout the year. And real estate markets value different amenities differently; swimming pools in Arizona vs. Alaska, for instance. The Lawrence Berkeley Labs study tells us how the California market values solar power... A similar study was conducted in 2010, that looked at homes in the San Diego area. This study arrived at a similar conclusion, still in California.
I would wager the Portland market values solar power a lot, probably as much as the California market, when adjusted for less power generation due to less sun. We aren't selling anytime soon, so the question is a bit academic anyway.
As an aside, I've been feeling our house is a bit cooler since we installed the solar panels on the roof last summer. Don scoffs, but it makes since to me that something else, not our roof, is absorbing or at least intercepting, the suns rays on the most exposed face of the roof, the house just might not get as hot. Speculation anyone?
Read the research report summary here.
And remember, the work on our project was done through Solarize Portland and the great folks at Imagine Energy.