Hey Mr. Green,

Most of the electrons we use at home are California solar, but we still have fossil “natural” gas for heating and hot water. We’d love to close the gas account. A heat pump could provide room heating and cooling, but heat-pump water heaters, the best electric option in terms of efficiency, still look poor relative to tankless gas. What do you recommend?

—Bruce in Pleasanton, California 

Well Bruce, if you have enough space left on your roof—or anywhere else—your ultimate solution is a solar water heater. It will help you get a whole lot of hot water (around 50 to 80 percent of your total or more) with very little additional gas (or electric) power to heat what the sun doesn’t.

You might not save any money with a solar system because of the high price. However, as the cost of gas or electricity rises, you may realize more savings. Go to the EPA website to find out if solar is feasible for you. It lets you calculate the cost of installation along with other considerations, such as location, available financing and incentives, the cost of conventional fuels (natural gas, oil, and electricity), and the cost of the fuel used for a backup water-heating system, if you want one. You can also simply request estimates from several local dealers.

You can get a federal tax credit of 30 percent for solar water heating until the end of 2019; after that, it drops to 22 percent by the time the rebate expires at the end of 2021. California also offers rebates of as much as $4,000 or more, while savings offered in all states can be found at energy.gov/savings/search.

Hot water production is the second-largest component of home energy consumption after space heating, accounting for 13 percent of total residential energy use. In the United States, this means that the average household spends $400 to $600 a year just to heat water.

For energy savings, it’s long been hard to beat tankless “instantaneous” heaters that only heat the water as needed, whereas “storage tank” models maintain a constant temperature, making tankless as much as 27 to 50 percent more efficient. The tankless models also have a longer life expectancy—20+ years, compared to 10 to 15 years for storage tank water heaters—and can be easily repaired to extend their life. But tankless models cost about three times as much, which means that the vast majority of houses still have the storage-tank models. There’s a newer possibility as well, with heat-pump models almost as efficient as tankless models, while costing about the same.

The size of your home might also influence your choice. For example, if your house is relatively small, a gas-fired tankless heater is probably the best choice. If your house exceeds 3,000 square feet, you could choose a larger heater with a heat pump. Good luck!