U.S. Department of Energy solar water heating Energy.Gov cost-effective way crunch some personalized numbers save up to 80% U.S. Department of Energy Estimate the cost of a solar water heater Calculating Solar Hot Water ROI Estimate the cost and energy efficiency U.S. Department of Energy Calculating Solar Hot Water ROI Estimate the cost and energy efficiency cost-effective way Calculating Solar Hot Water ROI some impressive numbers U.S. Department of Energy solar water heaters excellent page on its website Department of Energy crunch some personalized numbers Estimate the cost of a solar water heater hot water costs in half U.S. Department of Energy's website