http://googleblog.blogspot.com/2009/07/should-you-spring-clean-your-solar.html#!/2009/07/should-you-spring-clean-your-solar.html http://googleblog.blogspot.com/

http://googleblog.blogspot.com/2009/07/should-you-spring-clean-your-solar.html#!/2009/07/should-you-spring-clean-your-solar.html http://googleblog.blogspot.com/2009/07/should-you-spring-cle... indicates that solar panels mounted flat do indeed need to be cleaned, but panels mounted at a sufficient angle will be rinsed off adequately by the rain and do not seem to benefit measurably from manual cleaning.