If you read the specifications on various pieces of alternating current (AC) equipment, you are likely to see the term kilovolt-amps, or KVA. This is a unit of power consumption, like the kilowatt, though the KVA more exactly measures a device's use of electricity. Kilowatts and KVA, while similar, have important differences.
Watts and Volt-Amps
You measure electrical power in units of either watts or volt amps. For DC, or direct current, 1 volt of voltage times 1 amp of current equals 1 watt. Voltage and current are both steady, so this calculation is easy. For AC, or alternating current, defining a watt becomes complicated, as volts and amps oscillate at 60 hz and can peak at different times. For this reason, AC equipment ratings use volt-amps, which also multiplie volts and amps together, and it accounts for their phase difference.
Size of a KVA
A kilovolt-amp is 1,000 volt-amps, making it similar to a kilowatt, which is 1,000 watts. A refrigerator, for example, uses 300 watts or .3 kilowatts. Most AC appliances have a slightly higher KVA measurement than their kilowatt rating. The refrigerator's KVA rating is .38. The KVA and kilowatt numbers are different because of the nature of alternating current and the behavior of electric motors.
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The difference between kilowatts and KVAs is accounted for in a number called the power factor. In an ideal electrical device, the AC voltage and current waves are the same, and the power factor is one. In a real-world device, the voltage may peak before the current, lowering the power factor. If a device's KVA is more than its kilowatts, it consumes more KVA than the power it produces.
Use of KVA
Household appliances, industrial machines, power supplies and related equipment carry KVA ratings. The KVA, the kilowatt and the power factor, taken together indicate a device's efficiency. This becomes more important with powerful machines because it affects their effectiveness and operating costs.