According to the Environmental and Energy Study Institute (EESI), there are 1.5 million solar water heaters already in use in U.S. homes and businesses. Solar water heater systems can work in any climate and EESI estimates that 40 percent of all U.S. homes have sufficient access to sunlight such that 29 million additional solar water heaters could be installed right now. Why are only 1.5 million homes on solar? That's only 5 %!