Water heating can be a significant cost in the operation of multifamily residential buildings. On average, water heaters account for between 15 and 25 percent of the energy consumed by residents in multifamily dwellings, according to the Energy Information Administration (EIA), the independent statistical agency of the U.S. Department of Energy. For some multifamily residences, a solar water heating system, which uses the sun’s energy to preheat water entering an existing gas or electric water heater, may be a cost effective means to reduce monthly heating bills.