Definition: A volt-ampere (VA) is a unit of measurement for apparent power in an AC (alternating current) electrical circuit. Apparent power is the product of the root mean square (RMS) voltage and the RMS current. It is the total power supplied to the circuit, including both real (active) power and reactive power.

History/origin: The concept of apparent power and the unit volt-ampere arose with the development of AC power systems in the late 19th century. Engineers recognized that AC circuits involved both real power (measured in watts) and reactive power. The volt-ampere unit was introduced to quantify the total power supplied, encompassing both components.

Current use: VAs are widely used in electrical engineering to specify the capacity of electrical equipment such as transformers, generators, and uninterruptible power supplies (UPS). It is also used in specifying the power requirements of electrical loads and the capacity of power distribution systems.