Volt-amperes (VA) and watts (W) are both units used to measure electrical power. For larger values, the prefix "kilo-" (k), as in kVA and kW, is often used.
So, why are there two different units for power? This article explains the key difference: what aspect of power each unit represents.
W (Watts): Active Power (also called Real or True Power)
This is the power that performs actual work, such as creating heat, light, or motion. It is the power consumed by the resistive parts of a circuit.
VA (Volt-Amperes): Apparent Power
This is the total power in an AC circuit, calculated as Voltage × Current. It represents the vector sum of Active Power (W) and Reactive Power (VAR). Apparent Power is the total power that the power source must be able to supply.
For more information about active power, reactive power, and apparent power,
Please refer to "Difference between DC power and AC power".
Typically, VA is used to specify the output of AC power sources, while W is used for DC power supplies. This is because in DC circuits, the power factor is always 1, so VA and W are identical.
Unit Conversion
To convert VA to W, you must know the power factor (PF) of the AC load. The relationship is expressed by the following formula:
Watts (W) = Volt-Amperes (VA) × Power Factor (PF)
For example, if the power factor is 1.0 (or 100%), the reactive power is zero, and the apparent power equals the active power (VA = W). This is true for purely resistive loads, where the voltage and current waveforms are in-phase, such as incandescent light bulbs and heaters.
In contrast, devices with inductive components like motors (which create magnetic fields) have a lower power factor, typically between 0.5 and 0.8. Equipment with a built-in power factor correction (PFC) circuit can significantly improve this, achieving a power factor of 0.95 or higher.