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.