Watts / Amps / Volts Calculator (Single Phase)

Single-phase AC power is the most common form of electrical power delivered to homes and small commercial buildings in North America (120 V / 240 V) and internationally (230 V). Unlike DC circuits where power is simply P = VI, single-phase AC circuits require accounting for the power factor — the cosine of the phase angle between voltage and current waveforms — when the load contains inductive or capacitive elements.

There are three types of power in an AC circuit: real power (P), measured in watts, which does useful work such as heating or driving motors; reactive power (Q), measured in volt-amperes reactive (VAR), which is stored and released by inductors and capacitors; and apparent power (S), measured in volt-amperes (VA), which is the product of RMS voltage and RMS current as seen by the source.

The power triangle illustrates the relationship between these three quantities: S² = P² + Q², with P = S × cos(φ) and Q = S × sin(φ), where φ is the phase angle. The power factor PF = P/S = cos(φ) ranges from 0 (purely reactive load) to 1 (purely resistive load). A power factor of 1 means all apparent power does useful work; lower values mean the source must supply more current than the load actually uses for real work, increasing distribution losses.

For residential loads like incandescent lamps and electric heaters, PF ≈ 1. Motors, transformers, and fluorescent lighting typically have PF between 0.7 and 0.95. Industrial facilities are often penalized by utilities for low power factor below 0.95, incentivizing the installation of power factor correction capacitors.

This calculator is essential for sizing generators, inverters, UPS systems, transformers, and electrical panels. A generator rated at 10 kVA can supply at most 8 kW of real power to a load with PF = 0.8. Undersizing based on watts alone, ignoring the VA rating, is a common and costly mistake that leads to generator overloads and premature failure.

In household wiring, the 120 V / 15 A outlet is rated at 1,800 VA of apparent power. If the connected load has PF = 0.85 (typical for a refrigerator compressor), the available real power is only 1,530 W — a practical limit that must be respected when calculating circuit loading.

The phase angle output tells you by how many degrees the current lags (inductive load) or leads (capacitive load) the voltage. This is critical for troubleshooting power quality issues and for configuring automatic power factor correction systems.