According to The Appraisal Journal, home value increases by $20 for every $1 reduction in annual utility bills.  So a solar energy system that saves $200 per year would add $4,000 to the value of the home.

Investments in energy efficient home features, such as solar energy systems, have historically been valued based on how quickly the investment pays for itself. But such investments are home improvement projects that add value to the home, which can be recovered when the house is sold. 

For residential solar electric systems, this added value is often half of the homeowner’s out-of-pocket investment after claiming all incentives and tax credits.

For solar water heating systems, this added value is often equal to or greater than the out-of-pocket investment, resulting in immediate positive cash flow for the homeowner if the investment amount had been added to the mortgage.

Read the articles:

Evidence of Rational Market Valuations for Home Energy Efficiency, The Appraisal Journal, October 1998

More Evidence of Rational Market Values for Home Energy Efficiency, The Appraisal Journal, October 1999