Volt-ampere

A volt-ampere, in electrical terms, is the amount of apparent power in a circuit equal to the product of voltage and current. While equal to the electrical power measured in watts for Direct Current (DC) circuits the apparent power may differ from the real power for Alternating Current (AC) circuits, where voltage and current may be out of phase. The real power is also defined as apparent power multiplied by the power factor.^[1]

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 simultaneous 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. Apparent power is the vector sum of reactive power and real power, and will always be greater than or equal to either component value. For reactive loads (loads containing capacitance or inductance), apparent power is close to reactive power and is much greater than real power. By convention, the unit of measure for apparent power is the volt-ampere. For resistive loads, real power equals apparent power; incandescent lights, electric heating elements, and toasters are examples of nearly resistive loads. ^[2]

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 Uninterruptible Power Supplies (UPSs). The discrepancy between the VA capacity and the capacity in Watts is due to design limits where the VA rating is limited by the current generating capacity and the watt rating is limited by the power conversion capacity of the UPS. When powering e.g. computer equipment, which may represent a reactive load with a low power factor, care must be taken not to exceed either limit.^[3]

See also

• AC power
• Volt-amperes reactive

References

1. ^ How to Convert Watts to VA and kVA to kW Simplified. Powerstream.
2. ^ Ciletti, M. D., Irwin, J. D., Kraus, A. D., Balabanian, N., Bickard, T. A., and Chan, S. P. (1993). Linear circuit analysis. In Electrical Engineering Handbook, edited by R. C. Dorf. Boca Raton: CRC Press. (pp.82–87)
3. ^ Watt Ratings Differs From Volt Amp RatingsAPC