We can use some simple numbers to estimate the power production of the Hoover Dam. It has an average hydraulic head height of 160 m, the effective vertical distance that water from its reservoir drops in order to power its turbines. The gravitational potential energy of a liter of water (which has a mass of 1 kg) is therefore: |
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With rare exceptions, the entire 500,000 liter per second flow rate of the Colorado River flows through the dam's turbines—not over its spillway (the overflow route that accommodates times of heavy rains and flooding). The efficiency of a hydroelectric turbine can reach 0.80 (or 80%), so an average power production for the dam is approximately: |
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Our estimate of a power generating capacity of 630 MW is not far off from the long-term average quoted by the U.S. Department of the Interior of 480 MW. In any case, that is a lot of power! 
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Thermal energy is the basic form of energy that most power plants produce, whether they use coal, oil, or natural gas. The energy source generates heat that, in turn, is transformed into another kind of energy in order to do the work we desire. When gasoline is ignited in a car's piston engine, the burst of heat in the chamber expands the fuel-gas mixture to drive the vehicle's crankshaft. The boiling water of 19th century steam engines was heated by burning coal. Nuclear fission in a reactor heats up water that turns turbines, thereby generating electricity.
Some power sources do not generate heat directly as part of the generation process. Hydroelectric dams use gravitational potential energy from water to turn a turbine, generating electricity. Photovoltaic cells convert radiant energy from the Sun into electricity. Wind turbines spin as air passes across them, generating electricity. These sources are all called 
