Volt-ampere
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A volt-ampere in electrical terms, means the amount of apparent power in an alternating current circuit equal to a current of one ampere at an emf of one volt. It is dimensionally equivalent to watts. An example might be the purchase of an AC transformer rated in electrical terms by volt amperes.
Hence;
- 10 kVA = 10,000 watts capability (where the SI prefix k equals kilo)
- 10 MVA = 10,000,000 watts capability (where M equals mega)
While the volt-ampere and the watt are dimensionally equivalent one may find products rated in both VAs and Watts with different numbers. This is common practice on UPS's (Uninterruptible Power Supplies). The VA rating is the apparent power that a UPS is capable of producing, while the Watt rating is the real power (or true power) it is capable of producing. The difference between these two numbers is the reactive power. Reactive power arises due to the effects of capacitance and inductance of components in the load to be powered by the AC circuit. In a purely resistive load the apparent power is equal to the true power and the amount of VAs and Watts used would be equivalent. However, in more complex loads, such as computers (which UPSs are intended to power) the apparent power used (VAs) will be larger than the true power used (watts). The ratio of this difference is called the power factor.
[edit] See also
[edit] References
Calculating VA & Watts: http://www.powervar.com/Eng/ABCs/CalcVAWATTS.asp
