This paper reports a detailed study of residential solar water heating (SWH) in seven U.S. Sunbelt locations. TRNSYS computer model performance projections from the Solar Rating and Certification Corporation (SRCC) website were combined with local utility rate structures and fuel CO_2 content to evaluate economic and greenhouse gas emissions. As expected, in all but a few isolated cases, gas storage backup water heaters had lower operating costs than electric water heaters.As with the economic analyses, electric storage backup water heaters were found to be superior to gas in only a few situations for CO_2 emissions basis as well. No situation was found where electric backup offered lower CO_2 emissions and lower operating costs. However, if a single tank electric backup system is substituted for a dual tank gas backup SWH system, then both CO_2 emissions and operating costs are lower in San Diego, which is an important market because of the SWH pilot incentive program begun in 2007. This pattern was also found in Phoenix.
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