I'm a certified energy auditor with over 30 years in contracting. I have yet to find a photo-voltaic installation with under a 30 year payback. Solar hot water may make a 15 or 20 year payback, but that is out the window with the cost of natural gas backup going down. There is only one product "Solarwall" out of Canada that has a legitamite payback of under 15 years. The application is often hard to find, but when it does it works great. Even then we can't always find the right application.
We haven't really tapped the envergy conservation market. Most energy upgrades have 20 year or longer expected life and therefore can't be amortized very quickly. A real problem for commercial building owners.