We solar contractors generally disparage electric tank water heaters, except when used for solar storage. But sooner or later, solar is going to let you down, in cloudy weather or when it’s very cold, and you’ll need hot water for dishes, hands, cleaning and filling the pasta pot (who wants to wait for that cold water to boil?).
Whatever backup system you use to close the gap between solar and the American lifestyle, it has to be available all the time, not dependent upon the solar equipment at all, and capable of supplying your assessed need for hot water until the sun replenishes your tanks.
Consider putting a little tank of electrically heated water under your kitchen sink. Two, three, five gallons of water, kept hot all the time (you can have a switch if you can plan those solar outages an hour in advance), ready to fill a pot, wash your hands, feed the dishwasher (yes, the dishwasher has a heater, but it lengthens the cycle by about an hour waiting for it), and even draw a mug of water for tea, almost hot enough to brew the leaves, can be had for the price of some electric power and an upfront cost ranging from $500 to $1000 US. How long will it take you to pay that investment off? Not sure that’s the right question.
What you’re buying with your money is convenience. It’s not easy to calculate a payback on that. If you’re able, by means of two or so installed point of use heaters, to turn off your backup, whether it was a gas fired boiler, oil fired tank, or big electric tank, you may save enough energy from that idle system to give yourself a payback on the point of use heaters. But showers and laundry are not served by these relatively tiny devices, and you may need to use your backup hot water source just to keep those important services going. Go ahead. Tell me you wash all your clothes in cold water, all the time. Ok; I believe you. I don’t; and I don’t recommend it, unless you dry them thoroughly at rather high temperature. Don’t make the world a better place for all those bacteria, allergens and dust mites you want to remove from your clothes. Do you think you’re making them dizzy in the spin cycle?
For houses with multiple baths, long piping runs, and several occcupants, point of use water heaters can be a real convenience and an energy saver. I say if your hot water source is less than thirty feet from your faucets, tank insulation and a timer are your best tactics. You can decide whether a significant upfront investment and the privilege of turning off your backup source for the day balance for you economically and energy-wise.