Watts to Horsepower: How to Compare Engine and Electrical Power

Ever wonder how your vacuum cleaner compares to a lawnmower? Learn the conversion between Watts and Horsepower for appliances and engines.

Introduction

Horsepower is a marketing unit that happens to be a real one. It was invented to sell steam engines by comparing them to the animals they replaced, and it has outlived the animals, the steam engines and even the industry it was created for.

The watt is the modern unit, and converting between them is straightforward once you know which horsepower is meant, because there is more than one.

The Core Mathematics

Three definitions are in common use, and they differ by about 1.5 percent:

1 hp ≈ 745.7 W · 1 kW ≈ 1.341 hp

The gap between mechanical and metric horsepower is why the same car is sometimes advertised with two slightly different figures in different markets. A 150 PS car is 148 hp, and neither number is wrong.

Where the Horse Came From

James Watt needed a way to sell steam engines to mine owners who currently used horses, so he measured what a horse could actually do and priced his engines against it. His figure was based on a horse turning a mill wheel, working out at roughly 33,000 foot-pounds per minute, which is where 745.7 W ultimately comes from.

The figure was, if anything, generous. A real horse cannot sustain one horsepower all day, though it can exceed it briefly. Watt had a commercial interest in not overstating his engines' advantage, and the unit stuck.

The unit named after him is defined far more cleanly. A watt is one joule per second, which makes it a rate of energy transfer and ties it directly to every other unit in the SI system.

Power and Energy Are Different

This is the distinction that matters most and the one most often blurred. Power is the rate of doing work; energy is the total amount done.

A 2,000 W kettle draws power at that rate while running. Left on for one hour it consumes 2,000 watt-hours, or 2 kWh of energy, which is what appears on the bill. The watt describes the tap; the kilowatt-hour describes the bucket.

Car specifications quote power because it determines acceleration and top speed. Electricity bills quote energy because that is what was actually consumed. Our guide to kilowatt-hours and megajoules covers the energy side.

Familiar Power Figures

That cycling figure is worth pausing on. A fit cyclist sustains perhaps a quarter of a horsepower, and a professional sprinter can hit around 1,500 W for a few seconds, which is two horsepower. Watt's horse was, by human standards, formidable.

Electric Cars Changed the Convention

Electric vehicles are usually rated in kilowatts, partly because the motor is an electrical device and partly because the same unit describes charging rates. A 150 kW motor and a 150 kW charger use one unit for two very different things, which is convenient once you notice that both are rates.

Converting for comparison, a 150 kW electric motor is about 201 hp. Note that electric motors deliver their full torque from a standstill, so a given power figure produces noticeably quicker acceleration than the same figure from a combustion engine.

Quick Reference Conversion Table

Conclusion

Divide watts by 745.7 for horsepower, or multiply kilowatts by 1.341. Check whether a figure is mechanical or metric horsepower if precision matters, and keep power and energy separate: one is a rate, the other is a total.

Try our watts to horsepower converter for quick and accurate conversions.

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