Idea: Solar Energy Test Kit
I wish that I could put something the size of, say, a briefcase on my roof, leave it there for a month, bring it down, plug in a USB cord, and read "A solar water heater would have generated X% of your hotwater needs. A PV installation of X panels would have generated Y KwH."
Basically: a single solar cell plus a data logger. I see a market in Hawaii (small) and the Southwest mainland (big).
Make it cheap enough and sell it to the geek market even. I suppose for the geek market you'd have to add WiFi connectivity.