Single-Phase Power Calculator

Single-phase AC distribution predates three-phase by about a decade. Westinghouse's Great Barrington installation of 1886 — the first commercial AC system in North America — delivered single-phase 3000 V / 500 V / 100 V for lighting only. Single-phase induction motors required additional work: Nikola Tesla's first motor was two-phase (1887), and it took until 1893 for Charles Steinmetz to perfect the split-phase starting technique that made practical single-phase motors possible.

The US chose 120/240 V split-phase for residential service because of Edison's legacy 110 V DC distribution. When Westinghouse AC displaced Edison's DC systems after the 1893 Chicago World's Fair, utilities kept the 110 V appliance standard but added a second live conductor at 180° phase inversion — giving 240 V for electric ranges, water heaters, and clothes dryers while preserving the 120 V lighting ecosystem. Europe standardized later (1930s) on 220–240 V single-phase, skipping the split-phase complication entirely.

Single-phase motors are limited to ~7.5 hp practically because starting torque requires either a capacitor-start winding (power factor drops 5-8%), shaded-pole construction (inefficient but cheap), or electronic variable-frequency drives. Above 7.5 hp, three-phase almost always wins on efficiency, cost, and torque ripple. Refrigerators, well pumps, and window AC units all juggle these trade-offs within the single-phase envelope.

Enter RMS voltage, RMS current, and power factor. The tool computes apparent power S = V·I (VA), real power P = S·PF (watts), reactive power Q = S·sin(φ) (VAR), and phase angle φ = arccos(PF). For resistive loads like heaters and incandescent bulbs, PF ≈ 1 so P = S. For motors, SMPS, and ballasts, PF is typically 0.7–0.95.

Typical single-phase outlets: US 120 V / 15 A NEMA 5-15 = 1800 VA; EU 230 V / 10 A Schuko = 2300 VA. US split-phase 240 V ranges draw from both 120 V legs simultaneously — treat as one 240 V load. Everything runs client-side; no values leave your browser.