In 2005, 18,000 terawatt-hours of electricity were generated. With almost 9,000 hours in a year, that averages out at a constant 2 TW or so. Generating capacity is a lot higher than that, because there are peaks and troughs and no plants operate at their full output all of the time.
No analogy makes it easy to picture a terawatt. A thousandth of a terawatt, a gigawatt, is more comprehensible. It is the output of a fairly large power station: Sizewell B, one of Britain's largest nuclear power stations, has an output of about 1.2 GW; the Hoover Dam on the Colorado River can produce about 1.8 GW.
A megawatt is a thousandth of a gigawatt. It takes 3–5 MW to power most modern trains (or, if you feel flash, you can think of one as the power of two Formula One cars). A kilowatt is easily thought of as an electric fan heater.
Domestic energy consumption is measured in kilowatt-hours. In 2004, the highest per capita use of electricity was in Iceland, where it reached 28,200 kWh per year. In the United States it is about 13,300 kWh a year; 300 million Americans thus use about 400 GW of power. In Chile the per capita level is 3,100 kWh, in China 1,600 kWh, in India 460 kWh. The lowest level, in Haiti, is 30 kWh.
