Volt-Amperes (VA) and Watts (W) are both units of measurement for electrical power, but they represent different aspects of energy in AC circuits. Understanding the distinction is critical for selecting the appropriate power source capacity.
The Key Difference: Active Power vs. Apparent Power
Watts (W): Active Power
Also known as "Real Power" or "True Power," this represents the power actually converted into useful work, such as heat, light, or mechanical motion. It is the power consumed by the resistive load.
Volt-Amperes (VA): Apparent Power
This is the total electrical capacity required by the circuit, calculated as the product of Voltage (V) and Current (I). It is the vector sum of Active Power (W) and Reactive Power (var).
AC power sources are typically rated in VA because the internal components must handle the total current (Apparent Power), regardless of how much of that power is actually performing work (Active Power). Conversely, DC power supplies are rated in Watts because, in DC circuits, voltage and current are in phase, making VA and W identical.
For a detailed comparison of AC and DC concepts, please refer to our article: Difference between DC power and AC power.
Power Factor and Conversion
To convert between VA and W, the Power Factor (PF) of the load must be considered. The relationship is defined by the following formula:
Watts (W) = Volt-Amperes (VA) × Power Factor (PF)
- Resistive Loads (PF = 1.0): For devices like heaters or incandescent bulbs, voltage and current are in phase. The reactive power is zero, so VA equals W (Active Power = Apparent Power).
- Inductive/Capacitive Loads (PF < 1.0): For devices with motors or transformers, the power factor is typically between 0.5 and 0.8. In these cases, the required VA capacity will be higher than the W rating indicates. Modern equipment often includes Power Factor Correction (PFC) circuits to bring the PF closer to 0.95 or higher, optimizing efficiency.