Measuring Energy
Adapted with permission from Community Energy Workbook, Rocky Mountain Institute, 1995
Measuring Electricity
Watts describe the rate at which electricity is being used at a specific moment. For example, 100 watts describes the amount of electricity that a 100-watt light bulb draws at any particular moment.
Watt-hours measure the total amount of electricity used over time. Watt-hours are a combination of the how fast the electricity is used (watts) and the length of time it is used (hours). For example, a 100-watt light bulb, which draws 100 watts at any one moment, uses 100 watt-hours of electricity in the course of one hour.
Kilowatts and kilowatt-hours are useful for measuring amounts of electricity used by large appliances, such as refrigerators, and by households. Kilowatt-hours are what show up on your electricity bill. One kilowatt (kW) equals 1,000 watts, and one kilowatt-hour (kWh) is one hour of using electricity at a rate of 1,000 watts. New, energy-efficient refrigerators use about 1.4 kilowatt-hours per day, and about 500 kilowatt-hours per year.
Megawatts are used to measure the output of a power plant or the amount of electricity required by an entire city. One megawatt (MW) = 1,000 kilowatts = 1,000,000 watts. The average size of US power plants is 213 MW. A 1000 MW power plant is a large plant.
Gigawatts measure the capacity of large power plants or of many plants. One gigawatt (GW) = 1,000 megawatts = 1 billion watts. In 1990, if all US electrical generating plants were operating at full capacity at the same time, they would have produced 690 GW.
Measuring Energy
Btu. A British Thermal Unit is a measure of energy content, usually used to describe the energy content of fuels. Because a Btu is so small, energy is usually measured in millions of Btus. For example, a "therm" of natural gas, such as you would find on your gas bill, is 100,000 Btus of gas.
- 1 Btu = the amount of energy required to increase the temperature of one pound of water (which is equivalent to one pint) by one degree Fahrenheit, roughly the heat produced from burning one match.
- 1,000 Btu = four-fifths of the energy contained in a peanut butter and jelly sandwich.
- 100,000 Btu = one therm. Roughly the energy contained in 80 peanut butter and jelly sandwiches.
- 1 million Btu = 1.1 days of US per capita energy consumption.
- 277 million Btu = US per capita energy consumption in 1990.
- 1 quadrillion Btu = quad. The US consumes more than 80 quads annually.
