From Wikipedia, the free encyclopedia
Jump to: navigation, search
Apparent power is the magnitude of the vector sum (S) of real (P) and reactive (jQ) AC power vectors

A volt-ampere (VA) is the unit used for the apparent power in an electrical circuit, equal to the product of root-mean-square (RMS) voltage and RMS current.[1] In direct current (DC) circuits, this product is equal to the real power (active power) [2] in watts. Volt-amperes are useful only in the context of alternating current (AC) circuits (sinusoidal voltages and currents of the same frequency).

While both the volt-ampere and the watt have the dimension of power (time rate of energy), they do not have the same meaning. For example, the useful work produced by an AC motor is measured in watts, but the losses (mostly as heat) which dictate the rating of a transformer are measured in volt-amperes because they are practically independent of the power factor of the load.[3][4] Since the utility companies must size their distribution networks (particularly transformers) for apparent rather than real power, they sometimes pass these costs onto the consumers, who in some cases (typically only for industrial rather than household consumers) are required to pay the apparent power they consume (energy actually as power multiplied by time).[5]

Some devices, including uninterruptible power supplies (UPSs) (which contain transformers), have ratings both for maximum volt-amperes and maximum watts. The VA rating is limited by the maximum permissible current, and the watt rating by the power-handling capacity of the device. When a UPS powers equipment which presents a reactive load with a low power factor, neither limit may safely be exceeded.[6][dead link] For example, a (large) UPS system rated to deliver 400,000 volt-amperes (400 kVA) at 220 volts can deliver a current of 1818 amperes.

The usage of this unit is not recognized by the International System of Units as a derived unit or a non-SI unit accepted for use with the SI. This because even if reactive or apparent, the quantity the unit var represents is power; is a rate of energy transfer.[7]

See also[edit]


  1. ^ 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)
  2. ^ IEEE 100 : the authoritative dictionary of IEEE standards terms.-7th ed. ISBN 0-7381-2601-2, page 23
  3. ^ S.V. Kulkarni; S.A. Khaparde (2012). Transformer Engineering: Design, Technology, and Diagnostics, Second Edition. CRC Press. pp. 33–34. ISBN 978-1-4398-5377-1. 
  4. ^ Anthony J. Pansini (1999). Electrical Transformers and Power Equipment. The Fairmont Press, Inc. p. 19. ISBN 978-0-88173-311-2. 
  5. ^ George Shultz (1991). Transformers and Motors. Elsevier. p. 236. ISBN 978-0-08-051958-6. 
  6. ^ Watt Ratings Differs From Volt Amp Ratings APC
  7. ^ SI Brochure, 8th ed.: "Just as the quantity symbol should not imply any particular choice of unit, the unit symbol should not be used to provide specific information about the quantity [...]" Section 5.3.2.