Like your refrigerator or AC unit, what makes heat pump water heaters remarkably efficient is that they extract ambient heat from the air, said Jeff Harris of NEEA, which helps promote the technology. After early quality issues, today’s models come with industry-leading 10-year warranties, high customer satisfaction and the ability to heat water four to six times more efficiently than gas or electric resistance heaters. “It’s the equivalent of an LED lightbulb,” Harris said.

The average model sips just about 1,200 kWh per year, the equivalent output of two to three solar panels on your roof (or about $200 in average retail electric costs) in most of the United States. The technology is far less polluting than gas water heaters and furnaces in residential neighborhoods that emit dangerous levels of nitrogen dioxide (NO2) — linked to lung cancer, heart disease, diabetes, preterm birth and higher rates of childhood asthma — as well as 85 percent more planet-warming carbon dioxide.

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Two main types exist. First, there’s the standard vertical tank topped by a heat pump. These can be dropped in almost anywhere: A 120-volt model plugs into a standard wall outlet (and avoids expensive electrical panel upgrades), while a 240V model offers higher efficiency but requires a higher-voltage outlet (some give you the option to switch between them). The second option is a “split” system, with an outdoor compressor and an indoor tank. Most are hybrid: An electrical resistance element delivers hot water quickly if the heat pump can’t keep up.

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Installing a heat pump water heater still costs more: about $4,000 per appliance (roughly split between equipment and labor), according to the Advanced Water Heating Initiative (AWHI). That compares with about $2,500 for gas and $1,500 for electric resistance. (DIY installations are cheaper.)Ask The Post AIDive deeper

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In the Bay Area and Southern California, peak pricing and exorbitant electricity rates by investor-owned utilities can drive up the cost of electricity four to five times higher than equivalent energy from natural gas. That wipes out the roughly three- to fourfold efficiency advantage of heat pump water heaters.Ask The Post AIDive deeper

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Is it saving me money? The total cost was $7,500 (high, but expected for a premium water heater at Bay Area rates with permits and a condensate pump). If that was the end of it, I would not have recouped my investment. My modest cost savings and higher standard electricity rate would never have paid back the premium.

But I had a few advantages: a $2,000 federal tax rebate (now expired) and a $1,200 credit over two years for enrolling in San Francisco’s demand-response programs (allowing the utility to dial down consumption at peak times). Being on PG&E’s electric vehicle plan allows me to “recharge” my hot water when rates are low, at night or during off-peak hours (using my tank as a thermal battery), and we never run out of hot water. A mixing valve to keep water at 140 degrees Fahrenheit gives me an extra buffer.

Those conditions put my system in a position to pay for itself after two years. Rebates and tax credits were important (and annoying to fill out), shortening the payback from 15 years. But the key was lowering my electricity rates: My time-of-use plans mean I can take advantage of lower costs each day. Compared with gas, I’ll save more than $2,600 over the heater’s lifetime of 15 years (or maybe longer).