new study from Flagstaff Research commissioned by CALSSA finds that solar water heating with electric backup can reduce greenhouse gas emissions by 90% compared to traditional gas tank water heaters. This surpasses the performance of heat pump water heaters, which reduce emissions by 81% on average across California climate zones.
For multifamily properties, solar water heating produces more than twice as much bill savings as heat pumps. For residential systems, solar water heating systems reduce monthly bills while heat pumps increase them.
California policy makers are intent on electrification of water heating. CALSSA produced this research to demonstrate that solar water heating needs to be part of that effort. In California to date, the majority of solar water heaters installed have used gas backup heaters, but solar water heaters are also very effective when used with electric backup heaters. Solar water heating can be used both for fuel switching from gas to electric and to significantly increase the efficiency of existing water heating.
State agencies are implementing two new programs for water heating electrification. The Building Initiative for Low Emissions Development (BUILD) program targets new construction and will be run by the California Energy Commission. The Technology and Equipment for Clean Heating (TECH) program will address retrofits and will by run by the CPUC. In addition, CEC aims to use Title 24 building standards to encourage the construction of all electric housing. Because heat pumps increase energy costs for homeowners, solar water heating is often a better option.
Flagstaff Research evaluated thirteen water heating technology configurations for cost and performance using detailed simulation models for customer behavior and real world conditions in every region of the state.
Heat pumps are only efficient if they can heat water slowly. Because customers expect hot water to be recharged quickly, heat pumps make heavy use of electric resistance elements. This results in significant electricity usage during TOU peak periods, which increases costs and emissions. Solar water heating systems, in contrast, can almost entirely avoid reliance on electricity during peak periods.
Although the BUILD and TECH programs have very limited funding, CALSSA expects that they will provide a good structure that can be further funded after they are up and running. More immediately, homebuilders can get a lot of compliance credit by including solar water heating in new houses and apartment buildings.

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