Solar water heating systems can cut energy use by up to 55 percent compared with gas systems.

Solar water heating systems installed at U.S. Veterans Administration hospitals in San Antonio and Kerrville, Texas, are projected to save the two facilities a total of $18.3 million over a 19-year period compared with conventional systems, according to the U.S. Department of Energy.

The systems are used to preheat water for the hospitals' main boiler plant and for laundry use. A total of 320 solar panels are included in the two systems, which were installed by Johnson Controls Inc., Milwaukee, in 2002.

Solar water heating collects roughly four times as much energy per square foot and is about four times less costly than solar photovoltaic systems, according to Eneref Institute, Doylestown, Pa., an organization that researches and advocates sustainability. Solar water heating saves 55 percent more energy than gas water heaters and 75 percent more energy than electric heaters, according to data from the American Council for an Energy-Efficient Economy.

Solar thermal water heating systems circulate liquid through rooftop panels heated by the sun. The liquid, food-grade antifreeze or water, transfers the heat to storage tanks that feed heated water into the conventional hot water system.

Eneref recommends hiring a certified member of the International Association of Plumbing and Mechanical Officials or other qualified company to install a system.

It also recommends installing equipment that is certified by the Solar Rating & Certification Corp., which provides performance ratings, certifications and standards for solar thermal products. For information, go to