first of all lets post some common sense you are not going to save $100 on a 12,000.00 water heater solar installation. You will have to install a solar unit to supply more than just one unit which will cost a lot more. The only way one could possibly make it a prudent investment is getting a substantial credit with both the govt tax incentives and the providing power company, with out both of you are just being a good citizen for clean energy.