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Want to save the Earth? Easy, just buy a couple of ice trays. To the long list of human inventions that are wrecking global climate—the internal combustion engine, the industrial era factory—add the automatic ice maker.

Climate modelers have long known that households are far bigger contributors to global warming than most laypeople realize. For all the blame tailpipe emissions take for escalating  temperatures, homes and office buildings are actually the single largest contributor to greenhouse gasses. One key reason is the 100-plus million refrigerators in America’s 111 million households. According to the Department of Energy, the standard fridge sucks up about 8% of the electricity used by all homes—a pretty big share given the dozens of big and small appliances and electronics that are also drawing juice.

That energy gluttony has always made refrigerators prime targets for design improvements and most of the big manufacturers have made real progress in squeezing every last bit of efficiency out of their machines—especially since they know that cash-strapped consumers are paying closer attention than ever to energy-consumption ratings before making their purchasing decisions. The problem is, those ratings are not always terribly precise. In general, refrigerators will simply get a gross energy-use score, without anyone examining just which components in the overall machine are driving the numbers up or down. Ice makers have thus long gotten a pass, but analysts at the National Institute of Standards and Technology (NIST) recently decided to give them a closer look—and they got a surprise when they did.

According to the just-released findings, the average ice maker in the average fridge increases energy consumption by 12% to  20%—a whole lot of juice for an appliance that is in operation 24 hours a day from the moment you first plug it in till the moment you replace it a decade or more later. The reason that number was so unexpected was that the large majority of refrigerators are  refrigerator-freezer combinations anyway—which means they’re freezing water and making ice no matter what. So why should the simple business of automating the process be so energetically expensive?

The answer, it turns out, is the tiny motor inside the freezing system that’s used to release the bits of ice  from the mold and dump them into a tray. A motor that is designed to operate in so cold a setting needs an internal heater to keep it from freezing up, and heating elements require a lot of power—in this case, roughly three fourths of the total additional energy the ice maker uses.

Certainly, on the list of big things that are responsible for global warming, the icemaker ranks a good ways behind the coal-fired power plant, but averting climate catastrophe is often a game played in increments and inches, and every kilowatt hour helps. NIST is thus urging refrigerator manufacturers to look closely at the design of their icemakers, insisting that there are “substantial opportunities for efficiency improvements merely by optimizing the operations of the heaters.”

That appeal to reason, NIST officials hope, will be enough. But just in case it isn’t, the Department of Energy has announced that it intends to add 84 kilowatt hours to the efficiency rating of every refrigerator equipped with an icemaker. Consumers will feel that fact in the wallet—and if manufacturers don’t scramble to improve their numbers, they soon will too.