To reduce greenhouse gases emissions, promoting solar water heaters (SWHs) has become an essential national policy in Taiwan. To implement this policy effectively, the applicability of SWHs in different regions must be analyzed. Previous studies generally performed SWH benefit-cost analyses based on total annual solar radiation; however, this method may overestimate energy production benefits because, for an SWH, the solar energy captured today cannot be preserved. Therefore, this study proposes the concept of effective solar radiation (ESR), which is based on potential heat output estimated using tap water temperature and solar radiation in each region. The benefits of SWHs are then assessed based on the number of effective days and ESR, instead of using total annual solar radiation. A procedure is established to evaluate the applicability of SWHs in each region based on proposed benefit-cost analyses. Possible outcomes of a national SWH program are estimated. The sensitivities of essential factors, including collector efficiency, installation cost, and discount rate, are also analyzed. Analytical results show that the ratios of ESR to total annual solar radiation for regions in Taiwan are about 82-89%. The payback periods vary at 6-15 years for different regions and heater types being replaced. The national program is expected to reduce greenhouse gases emissions by approximately 150,000 tons eCO 2 annually.