Energy and Power Units: The Basics

Electrical energy is less intuitive than mechanical energy because it acts invisibly. The closest analog to lifting a heavy suitcase is the force you feel when you play with magnets.

Electrical energy is based on the attraction and repulsion of charged particles, i.e., the electromagnetic force. The strength of the charges and the distance between the particles combine to create an electric potential difference or voltage. In electrical applications, voltage pulls electrons through a conductor to create a current, not unlike gravity pulling water molecules through a pipe.

The standard unit of electric charge is the coulomb (C). Charles-Augustin de Coulomb (1736 - 1806) was a French physicist who discovered the relationship between electric charges, distance and force. A coulomb is the amount of charge transported by a current of one ampere in one second (C = A · s) and is a surprisingly large unit. The repulsive force between two +1 coulomb charges held one meter apart is 9 x 10 9 N, or more than a million tons! Thus, charge is most often measured in micro- or nanocoulombs.

The standard unit of electric potential is the volt (V), in honor of Count Alessandro Volta (1745 - 1827), known for his development of the electric battery. A volt is equivalent to one joule of energy per coulomb of charge (V = J / C). Household electric service in the U.S. is typically 110 V, although 220 V may be used for heavy appliances. A common flashlight battery delivers 1.5 V, while lightning can be around 100 MV. Long-distance transmission lines operate at 110 to 1,200 kV.

The standard unit of electric current is the ampere (A), or amp. French physicist André-Marie Ampère (1775 - 1836) was one of the main discoverers of electromagnetism. One ampere equals the displacement of one coulomb of charge per second (A = C / s). Most household circuits pull less than 15 A.

Most electric power is produced by burning fossil fuels. PV, wind turbines and other technologies offer clean, renewable alternatives, but they have a long way to go to replace existing generating plants. In 2006, fossil fuel-burning plants in the U.S. generated 2,874 billion kWh, and nuclear plants generated 787 billion kWh. All renewable-energy sources put together generated 385 billion kWh, less than 10 percent of total U.S. production.

Part of the challenge is a matter of scale. A large oil-, gas- or coal-burning plant cranks out 2 to 3 GW at full capacity. Most concentrating solar installations generate tens of megawatts, while an up-to-date wind turbine generates around 3 MW. The proposed Cape Wind project needs 130 turbines to provide just three-quarters of Cape Cod's electricity. A typical grid-tied home photovoltaic system produces less than 6 kW.

On the other hand, plenty of renewable energy is available if we just can figure out how to put it to use. The amount of energy in the sunlight that falls on one square meter of the Earth's surface is roughly one kW per second, or 3,600 kW per hour. Refrigerators and toaster ovens pull 1.0 to 1.5 kW each. Incandescent light bulbs draw from 40 to 150 W, while CFLs deliver the same amount of light at 10 to 40 W. Altogether, an average U.S. home uses about 1,000 kWh per month, a tiny fraction of the sun power that hits its roof.