Horsepower Calculator — AC Single Phase Power Supply

This calculator ties together the five electrical variables of a single-phase AC motor — horsepower, voltage, current, efficiency, and power factor — through one relationship. Enter any four and it returns the fifth, so you can find the current a motor will draw, the horsepower a measured current implies, or the efficiency or power factor behind a set of readings.

The formula

HP = E · I · EFF · PF / 745.7. Reading it left to right: voltage times current is the apparent power in volt-amps; multiplying by the power factor gives the real power the motor actually draws in watts; multiplying by the efficiency gives the mechanical power at the shaft; and dividing by 745.7 watts per horsepower converts that to HP (some references round the constant to 746). Switch to metric and the output is divided by 1000 to read kilowatts instead. Efficiency and power factor are entered as decimals between 0 and 1 — for example 0.85, not 85.

Solve in any direction

• Current — the most common use: how many amps a known motor will pull, for wire and breaker sizing (confirm against the NEC full-load current tables).
• Horsepower — the shaft power implied by a measured voltage and current.
• Efficiency or power factor — backed out from the other four; a solved value above 1.0 means the numbers are inconsistent.

Single phase only

This applies to a single-phase supply. A three-phase motor uses the same idea but with an extra √3 factor (HP = √3 · E · I · EFF · PF / 745.7), so do not use single-phase figures for three-phase wiring. For the pump side of the motor, see pump power and brake horsepower; to size the motor with service factor and margin, use motor sizing.

Worked example

A 230 V single-phase motor drawing 10 A at 0.85 efficiency and 0.85 power factor delivers HP = 230 · 10 · 0.85 · 0.85 / 745.7 ≈ 2.23 hp (1.66 kW). Along the way it draws 2300 VA of apparent power and 1955 W of real power, of which 1662 W reaches the shaft.