Why are the Required Watts and VA So Different?
For complex loads (such as inductors and capacitors), the voltages and currents present may be out of phase. When they are out of phase, the maximum volts and amps in the circuit when multiplied together gives the VA. The Watts, or power dissipated by the circuit, may be less than the VA due to the phase difference. The VA is always greater than or equal to the Watts. For example, when using a purely resistive load, such as a resistor, the Watts and VA are the same because the voltage and current are in phase. However, when using a complex load, such as a motor, the voltage and current are out of phase and the VA will be much larger than the Watts.
Watts is a scalar quantity which is frequently used to measure system efficiency. It is the energy supplied by the utility company over a given period of time and is commonly referred to as power. Except for heavy industrial users, the utility company only bills users for the watts consumed. Watts are directly convertible into mechanical work or BTUs (British Thermal Units) of heat. Wasted power is paid for a second time in terms of additional loading on the user's air-conditioning system. Mathematically, it is a scalar quantity resulting from the vector product of two vector quantities (volts and amps). It is NOT the simple algebraic product of the rms volts times rms current.
VA on the other hand IS the scalar quantity resulting from multiplying the magnitudes (rms) of the vector quantities (volts and amps). This resulting quantity will never be smaller than the watts demanded by an instrument. Uninformed users incorrectly use VA to assess the device's over-all efficiency and power demands. VA is most frequently and correctly used by electricians to determine proper ac mains conductor gage and circuit breaker sizing.