When you're trying to match a sound system's output to speakers, you don't want to under-power the speakers, but you don't want to overpower them, either. A watt is the amount of work done by one volt moving one ampere (often called an "amp" of current) of electricity. A speaker's wattage is a description of its strength. Knowing the voltage of a power speaker and how many amps its circuitry is designed to carry, you can calculate the speaker's wattage using Ohm's Law, which describes the relationship between volts, amps and watts.
Read the printed information provided with the speaker. Determine what voltage the speaker requires for it to operate.
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Investigate the speaker's amperage (the number of amps). It's either printed on a plate on the speaker or in the speaker's printed information.
Multiply the number of volts by the number of amps to determine how many watts the speaker uses. If the speaker operates at 110 VAC, at 2 amps, the speaker's wattage is 220 watts.