AC Wattage Calculator
Find real power in an AC circuit.
AC real power accounts for the power factor between voltage and current.
How the Math Works
The AC Wattage Calculator uses the fundamental power equation P = V·I·PF, where P represents real power in watts, V is the RMS voltage in volts, I is the RMS current in amperes, and PF is the power factor (a dimensionless number between 0 and 1). Real power accounts for only the portion of apparent power that performs useful work, with the power factor adjusting for phase differences between voltage and current waveforms in AC circuits. When the load is purely resistive, PF equals 1, but inductive or capacitive loads like motors and transformers reduce the power factor below 1, making this correction essential for accurate power measurement.
Practical Applications
To use this calculator practically, measure or obtain the RMS voltage across your load using a multimeter, determine the RMS current flowing through the circuit with an ammeter, and find the power factor from your device's nameplate, specifications, or by using a power analyzer. For example, a 120V motor drawing 5A with a power factor of 0.85 requires 120 x 5 x 0.85 = 510W of real power. This calculation is essential for proper electrical sizing, ensuring circuit breakers and wiring can handle the actual power load rather than just the apparent power, and for accurate energy billing since utilities charge for real power consumed.
Day-to-Day Use
Understanding your appliance's real power consumption helps you make informed decisions about energy usage and costs. By calculating actual wattage, you can determine how much your devices cost to run per hour, compare efficiency between different appliances, and identify energy hogs in your home or office. This knowledge is particularly valuable when sizing backup generators, selecting appropriate UPS systems for computers, or installing dedicated circuits for high-power equipment like air conditioners or workshop tools, ultimately saving money and preventing electrical overload hazards.
Worked example
230 V, 5 A, PF 0.9 → 1035 W.
FAQ
What if PF = 1?
Then P = V·I, the same as DC power.