Volt-ampere

This article may be too technical for most readers to understand. Please help improve it to make it understandable to non-experts, without removing the technical details. (December 2019) (Learn how and when to remove this template message)

volt-ampere
Apparent power (${\displaystyle S}$) is the magnitude of the vector sum of real power vector (${\displaystyle P}$) and reactive power vector (${\displaystyle Q}$); ${\displaystyle \varphi }$ is the phase shift between voltage and current.
General information
Apparent power
Symbol	V⋅A
Conversions
1 V⋅A in ...	... is equal to ...
SI base units	1 kg⋅m2⋅s−3

The volt-ampere (

Formulation

For a simple

${\displaystyle P=I\cdot V}$

For alternating current, both the voltage and current are oscillating. Instantaneous power is still the product of instantaneous current and instantaneous voltage, but if both of those are ideal sine waves driving a purely resistive load (like an incandescent light bulb), average power becomes (with subscripts designating average (av), peak amplitude (pk) and root mean square (rms)):

${\displaystyle P_{\text{av}}={I_{\text{pk}}\cdot V_{\text{pk}} \over 2}=I_{\text{rms}}\cdot V_{\text{rms}}}$

More generally, when voltage and current are not in phase, these products no longer represent average power but a new apparent power (S), measured in volt-amperes:

${\displaystyle S=I_{\text{rms}}\cdot V_{\text{rms}}}$

Usage

The relationship between real power and apparent power is described by the power factor. With a purely resistive load, they are the same: the apparent power is equal to the real power. Where a reactive (capacitive or inductive) component is present in the load, the apparent power is greater than the real power as voltage and current are no longer in phase. In the limiting case of a purely reactive load, current is drawn but no power is dissipated in the load.

Some devices, including

VA ratings are also often used for transformers; maximum output current is then VA rating divided by nominal output voltage.[5] Transformers with the same sized core usually have the same VA rating.

The convention of using the volt-ampere to distinguish apparent power from real power is allowed by the SI standard.^{[6][page needed]}

Volt-ampere reactive

In electric power

Special instruments called varmeters are available to measure the reactive power in a circuit.[7]

The unit "var" is allowed by the