Other energy unitsAs said before, physicists prefer to use the joule, or watt second, as their energy unit.This is the standard unit for scientific purposes.Another unit is the erg, equivalent to one ten-millionth (0.0000001) of a joule. Thisis said to be roughly the amount of energy needed by a mosquito to take off after it hasbitten you (not including the energy needed for the bite itself). The erg is used in labexperiments involving small amounts of expended energy.You have probably heard of the British thermal unit (Btu), equivalent to 1055joules. This is the energy unit most often used to indicate the cooling or heating capac-ity of air-conditioning equipment. To cool your room from 85 to 78 degrees needs a cer-tain amount of energy, perhaps best specified in Btu. If you are getting an airconditioner or furnace installed in your home, an expert will come look at your situa-tion, and determine the size of air conditioning/heating unit, in Btu, that best suits yourneeds. It doesn’t make any sense to get one that is way too big; you’ll be wasting yourmoney. But you want to be sure that it’s big enough—or you’ll also waste money because of inefficiency and possibly also because of frequent repair calls. Other energy units332-7Hypothetical graph showing the powerconsumed by a typicalhousehold, as a function of the time of day.