By Bert Markgraf – Updated March 24, 2022
When voltage and current are in phase, kilovolt‑amperes (KVA) equal kilowatts (KW) – the real power used by a circuit. If they’re out of phase, the KVA value rises above the KW, representing apparent power that must be multiplied by the power factor to obtain the true KW consumption.
In a purely resistive AC load, voltage and current rise and fall together, producing real power. Under these conditions, multiplying the instantaneous voltage by the instantaneous current and dividing by 1,000 gives KVA, which is numerically identical to KW. This is why most residential appliances list their power rating in kilowatts.
Industrial motors, transformers, and other inductive loads often create a lag between voltage and current. The motor’s magnetic field delays the current, so although the same voltage and current values exist, the circuit’s real power is lower than its apparent power. Consequently, the KVA figure – calculated from the raw voltage and current – exceeds the KW. To determine the usable KW, the apparent KVA is multiplied by the power factor (a decimal between 0 and 1). Typical power factors for large industrial equipment hover around 0.8, meaning 80% of the apparent power is actually used to perform useful work.
Consider a steel mill that operates a heavy rolling motor. The motor may deliver 80 KW of useful power while the voltage and current in its circuit are out of phase, yielding an apparent power of 100 KVA. Utilities charge customers based on the KVA supplied, so the mill pays for the full 100 KVA even though only 80 KW is converted to mechanical energy.
By installing power‑factor correction devices – typically capacitors or synchronous condensers – the mill can shift the current phase closer to the voltage. When the phase alignment improves, the apparent power drops to match the real power, reducing the customer’s bill to 80 KVA for the 80 KW of motor output.
Because utilities must supply the complete voltage and current regardless of phase alignment, they base charges on KVA. Only the in‑phase component actually delivers useful power; the rest is simply circulating within the circuit.
Residential loads are predominantly resistive, so appliances such as stoves, ovens, toasters, and electric heaters operate with voltage and current in phase, producing real power measured in KW. Even motors in household appliances (refrigerators, washing machines, dryers) are small enough that the utility’s billing remains tied to KW, or they already incorporate internal power‑factor correction. As a result, homeowners typically pay only for the real power consumption, not for the apparent power.
