Heat pump water heaters are what you're looking for. They're still tanked so don't need as much instantaneous power to heat a large body of water which also makes them amenable to batteries as a backup. As a bonus, in California water heaters are often installed in garages for townhomes and detached houses and the heat pump will end up cooling the surrounding space which tends to be a benefit during the summers and isn't a big problem through our generally mild winters.