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Volt-ampere |
A volt-ampere in electrical terms, is the amount of power in an alternating current (AC) circuit equal to a current flow of one ampere at an electromotive force of one volt. It is equivalent to watts for non-reactive circuits and in the strictest sense is identical, but by convention the two are used as units for subtly different physical quantities.
The instantaneous real power delivered to any load is the voltage across the load at a given instant times the current through the load at that same instant. Making such a measurement requires an instrument that can make simultameous instantaneous measurements of current and voltage. If separate current and voltage instruments that are incapable of coordinating their measurements are used, and which give root mean square measurements, and the two measurements are multiplied, the result will be apparent power; for reactive loads (loads containing capacitance or inductance), apparent power is greater than real power. By convention, the unit of measure for apparent power is the volt-ampere. For resistive loads (i.e. non-reactive loads) real power equals apparent power; incandescent lights are an example of nearly resistive loads. (Ciletti et al. 1993, 82)
While the volt-ampere (abbreviated VA) 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. In the type of loads often used with UPSs, such as computers, the apparent power used (volt-amperes) will be larger than the true power used (watts). The ratio of these two quantities is called the power factor.
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