Water is measured in gallons, eggs in dozens, meat in pounds. Electricity cannot be poured into a container or weighed on a scale, but is rather a thing that is thought of as always in motion.

So we must think of electricity as flowing past a given point over a period of time. The rate at which electricity flows, or current, and the pressure under which it flows, voltage, are combined to arrive at a unit of measurement for electricity: watts.

Current: An electric current is the flow of electrons past a given point over a period of time. In the same way that we can think of the flow of water in terms of gallons per minute, we can think of the flow of electricity in terms of coulombs per second. The coulomb is the standard measure of electric charge. Many people are unfamiliar with the term coulomb, because we generally use the word amp (ampere) instead, which is defined as coulombs per second.

Voltage: Water, air, and other substances can be put under pressure, which we generally measure in pounds per square inch. Electricity can also be put under pressure, which is measured in volts. In the United States, most house wiring is at 120 volts for lighting and 240 volts for permanently installed appliances and motors that run machinery.

Watts: The total amount of power in a circuit is measured in watts. Amps alone or volts alone don’t tell us the amount of actual power. They must be considered together. The amount of power is volts and amps taken together: Watts is volts multiplied by amps. W = V × A.

This formula is always correct with direct current but correct part of the time with alternating current. For lamps, toasters, ranges, and other heating devices, it is correct. For motors, loads with transformers such as radios and TVs, and ballasts (fluorescent lights) the watts are a little less than volts × amps.

Watts measure power exactly as horsepower does: 1 hp = 746 watts. A single watt is a very small amount of power. Generally, watts are discussed on a larger scale: kilowatt, which is a thousand watts.

Kilowatt-hours: To measure the rate at which power is being used over time, we say kilowatt-hours, or watt-hours. Electrical energy from a power supplier is measured and paid for by the kilowatt-hour.

Operating cost per hour: To find out how much it costs to run an electrical load per hour, simply multiply the watts the item consumes by the going cost per kilowatt-hour in your region. For example, operating a 60-watt lightbulb in Los Angeles, where residents pay an average of $0.28 per kilowatt-hour, would cost a little less than two cents per hour: 60 W/1000 = 0.06 kilowatts 0.06 kW × $ 0.28 = $ 0.0168 The accompanying chart may aid you in estimating the power required (and the operating cost) for standard pool equipment.