Horsepower and Electric Motors: From Nameplate to Breaker
By the VoltConvert team · June 12, 2026
Horsepower refuses to die. Two and a half centuries after James Watt coined it to sell steam engines against literal horses, every pump, compressor and pool motor in a US hardware store still wears an HP label. Here's how to translate it into the electrical numbers that actually matter.
746 watts, and which horsepower you mean
One electric (imperial) horsepower is defined as 746 watts — Watt's 550 foot-pounds per second, converted. Europe complicates things with metric horsepower (PS) at 735.5 W, about 1.4% smaller, which mostly haunts vehicle spec sheets. For motors in the US, UK and most industrial contexts, 746 is your number, and the HP to watts / watts to HP calculators handle both directions.
The crucial fine print: HP is output, not consumption
A 1 HP motor delivers 746 W of mechanical power at the shaft. It does not consume 746 W of electricity — no motor is 100% efficient. Typical efficiencies run 80–92%, so that 1 HP pump actually draws 810–930 W from the wall, plus a power-factor penalty on current. This single misunderstanding explains most "why is my generator overloaded" mysteries: a shed full of '5 HP total' equipment is electrically a 4.5–5 kW load, not 3.7 kW.
From HP to amps: the full formula
Current = (746 × HP) ÷ (V × PF × efficiency) for single phase, with a √3 joining the denominator for three phase. A 5 HP single-phase compressor at 240 V, PF 0.85, efficiency 0.88: about 20.8 A running. The HP to amps calculator exposes every term so you can see what moves the answer. For US code compliance note that the NEC generally directs you to its own current tables (430.248/250) or the nameplate FLA for sizing — the formula tells you why those tables say what they say.
Starting current: the 6× ambush
An induction motor started direct-on-line briefly draws 5–8 times its full-load current while spinning up. That 20 A compressor demands 100–160 A for a second or two. Breakers and fuses for motor circuits are deliberately sized and typed to ride through this (time-delay fuses, HACR-type breakers), and generators powering motors need surge headroom far beyond running watts. If a portable generator stalls every time the well pump kicks in, starting current is the culprit — not the running math.
Buying motors like a professional
Read three numbers, not one: HP (the work it can do), FLA on the nameplate (the current it actually draws), and the service factor (how much brief overload it tolerates). Compare energy cost using input watts, not HP — at 8 hours a day, the efficiency gap between a cheap motor and a premium one pays the price difference inside a couple of years. And when replacing a gas engine with electric, remember electric torque behaves differently: a 2 HP electric motor often replaces a 3.5–4 HP gas engine in practice.
Keep the chain in your head — HP → watts out → watts in → amps → breaker — and motor circuits stop being mysterious. The calculators hold the formulas; the judgment about efficiency, duty and surge is the part worth learning.