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 equivalent to
watts for non-reactive circuits. 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
UPSs (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, as opposed to
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 (
incandescent lights for example), 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 these two quantities is called the
power factor.
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