Page 53 - Teach Yourself Electricity and Electronics

Other energy units 33

2-7 Hypothetical graph
showing the power
consumed by a typical
household, as a function
of the time of day.

Other energy units

As 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. This
is said to be roughly the amount of energy needed by a mosquito to take off after it has
bitten you (not including the energy needed for the bite itself). The erg is used in lab
experiments involving small amounts of expended energy.
You have probably heard of the British thermal unit (Btu), equivalent to 1055
joules. 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 air
conditioner 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 your
needs. It doesn’t make any sense to get one that is way too big; you’ll be wasting your
money. 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.