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 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 volt-amp refers to the maximum power flow, while the watt refers to a time-averaged power flow. The power flow varies as a sine function.
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. 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.
