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How Green Is The A/c, Anyway?

Does it take less energy to cool a home than heat it? In other words, is running the air conditioner (relatively) better for the planet than cranking up the thermostat? A short piece in Wired magazine's recent batch of counterintuitive green articles came down in favor of A/C, which would imply that living in warmer climes like Arizona is less-energy intensive—at least in this one respect—than living in the wintry Northeast. But now Ray Humbertpierre of the University of Chicago says that Wired didn't get things quite right:

But wait, the story doesn't stop there. First, there's the fact that air conditioning almost invariably runs off of electricity, and the increased electricity demand is a big source of the pressure to build more coal-fired power plants. A house can be heated by burning natural gas, and right there air conditioning becomes 1.8 times worse than heating, because natural gas emits only 55% of the carbon of coal, per unit of heat energy produced. And it gets even worse: Coal fired power plants are only 30% efficient at converting heat into electricity, on average, so there you get another factor of 3.3 in carbon emissions per unit of energy transferred between the house and its environment.

Finally, figure in a typical electric line transmission loss of 7% and you get another factor 1.075. Put it all together with the energy efficiency of the air conditioner itself and air conditioning comes in at a whopping 2.19 times less efficient than heating. for a given amount of temperature difference between house and environment. That means that so far as carbon emissions go, heating a house to 70 degrees when the outside temperature is 40 degrees is like cooling the same house to 70 degrees when the outside temperature is 83.7 degrees.

And that's still not the end of the story. A house in need of air conditioning has other heat inputs besides the heat leaking in from outside, and all that extra heat needs to be gotten rid of as well....

Er, so it's all very complicated, and I don't envy anyone trying to estimate whether they should stay in Boston or move to Phoenix to save on heating and cooling costs (not that there are such people, but if there were...). Of course, if Congress ever got around to passing a carbon tax or cap-and-trade bill, this would all become easy: The more carbon-intensive lifestyle would simply be a lot costlier than lower-carbon alternatives. That's the beauty of carbon pricing, at least in theory, though admittedly it will make life duller for journalists filing stories on whether it's better to buy a used car or get a brand-new Prius…

--Bradford Plumer