Single-Phase Power Calculator
P, S, Q, I for a single-phase AC circuit.
How to Use
- Enter V, I (or P), PF.
- S = V·I. P = S·PF. Q = √(S²−P²).
Show Work
Formulas
History of Single-Phase Distribution
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.
About This Calculator
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.
About the Single-Phase Power Calculator
The Single-Phase Power Calculator is a free tool for electronics and circuit design. It runs right in your web browser, so there is nothing to download. P, S, Q, I for a single-phase AC circuit.
How it works
Type your numbers into the boxes. The answer shows up right away — you do not have to press a button. If you change a number, the answer changes too. So you can try different numbers and watch what happens, or check an answer you worked out yourself. Just make sure each box has the right kind of number in it.
Want the deeper story? The Knowledge Base explains the ideas behind the tools in more detail.
Frequently Asked Questions
Neutral current?
Equals line current in single phase.
PF = 1?
Pure resistive — P = S.
How do I use the Single-Phase Power Calculator?
Simply type your numbers and read the result, which refreshes the instant you change something. There is nothing to submit and nothing to wait for.
Is it free? Does it work without internet?
Yes to both. It is free with no sign-up, and once the page has loaded it keeps working even with no internet.
Where does my data go?
Nowhere — every calculation runs on your own device. Nothing you enter is uploaded, logged, or stored.
Common Use Cases
Residential
120/240 V split-phase.
Appliance
Nameplate conversion.
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