About the Watts to Horsepower Calculator
Power is measured in watts for electric motors, generators, appliances and electric cars, and in horsepower for engines, pumps, lawnmowers, boats and older machinery. Comparing an electric motor with a petrol engine, reading a car's specification from another market, or choosing a pump or generator often means converting watts or kilowatts into horsepower. There is a complication: three different horsepowers are in common use.
This watts to horsepower calculator converts watts into mechanical horsepower, the most common kind, and also shows metric horsepower (PS), electrical horsepower and kilowatts, with the energy used by running at that power for an hour.
How to Use the Watts to Horsepower Calculator
Enter the power in watts. For a figure in kilowatts, multiply by 1,000 first: 150 kW is 150,000 W.
The horsepower appears straight away, with the other horsepower units and kilowatts.
The Formula
mechanical hp = watts ÷ 745.699872 (550 foot-pounds per second)
metric hp (PS) = watts ÷ 735.49875 (75 kilogram-force metres per second)
electrical hp = watts ÷ 746 (a round figure for motors)
kW = watts ÷ 1,000Step-by-Step Example
A 1,000 watt motor.
Mechanical: 1,000 ÷ 745.7 = 1.34102 hp
Metric: 1,000 ÷ 735.5 = 1.35962 PS
Electrical: 1,000 ÷ 746 = 1.34048 hpSo a 1 kW motor delivers about 1.34 horsepower.
A 150 kW car engine is 150,000 ÷ 745.7 = 201.153 hp, or 203.943 PS — which is why European brochures would call it a 204 PS engine.
Quick Reference
100 W = 0.134 hp 10 kW = 13.41 hp
250 W = 0.335 hp 50 kW = 67.05 hp
500 W = 0.671 hp 100 kW = 134.10 hp
746 W = 1.000 hp 200 kW = 268.20 hp
1,500 W = 2.012 hp 500 kW = 670.51 hpThree Kinds of Horsepower
Mechanical horsepower comes from James Watt, who in the 1780s compared his steam engines with the work of mill horses and settled on 33,000 foot-pounds per minute, or 550 per second. That is 745.7 watts, and it is the horsepower used for cars in the United States and the UK.
Metric horsepower, called PS in Germany and CV in France, Italy and Spain, was defined when those countries went metric as 75 kilogram-force metres per second, 735.5 watts. It is about 1.4 percent smaller, so the same engine has a slightly larger PS figure than hp figure.
Electrical horsepower is a round 746 watts, adopted by electric motor makers to simplify ratings. It is almost identical to mechanical horsepower.
Motors and Efficiency
A motor's horsepower rating usually describes its mechanical output. Because motors lose some energy as heat, the electrical power drawn is higher. A 1 hp motor that is 85 percent efficient draws about 746 ÷ 0.85 = 877 watts from the supply. When sizing a generator or circuit for a motor, use the input power, and allow extra for the surge when the motor starts.
Electric and Petrol Cars
Electric car motors are rated in kilowatts. A 100 kW motor is about 134 hp, similar to a family petrol car; high-performance electric cars exceed 300 kW, over 400 hp. Petrol engines quote peak power at a particular engine speed, while electric motors deliver strong power over a wide speed range, so equal horsepower figures can still feel different on the road.
People, Animals and Machines
A healthy adult can sustain about 75 to 100 watts of useful work, roughly an eighth of a horsepower, and trained cyclists can hold 300 to 400 watts, about half a horsepower, for an hour. A real horse can produce well over one horsepower in short bursts. A small garden mower might have a 2 to 3 kW engine, about 3 to 4 hp.
Generators and Power Tools
Generators, power tools and pumps are sold with ratings in either unit, and choosing equipment often means comparing them. A 5 hp petrol engine driving a generator produces about 3.7 kW of mechanical power, and after generator losses might deliver around 3 kW of electricity. A 1.5 hp pond or pool pump draws roughly 1.1 kW of mechanical output, and more from the socket. Workshop tools such as table saws and compressors often state motor power in horsepower in the US and in watts elsewhere, so the same model can appear with quite different-looking figures in two catalogues.
Converting Back
To turn horsepower into watts, multiply by 745.7, or by 746 for electrical ratings. A 2 hp motor is about 1,491 watts, and a 300 hp engine about 223.7 kilowatts.
Quick Mental Rule
For everyday estimates, three-quarters of a kilowatt is one horsepower, so multiply kilowatts by 4 and divide by 3: 90 kW is about 120 hp, against the exact 120.7.
Understanding Your Result
The headline is the power in mechanical horsepower.
The metric horsepower line gives the PS figure used in many European specifications.
The electrical horsepower line gives the figure used for electric motor ratings.
The kilowatts line gives the power in kW and the energy used by running for one hour.
When Should You Use This Calculator?
Use it to compare electric motors with petrol and diesel engines.
Use it for car, motorcycle and boat specifications from different markets.
Use it to size pumps, generators, compressors and garden machinery.
Use it for physics and engineering problems involving power.
Common Mistakes
Mixing hp and PS. Metric horsepower is about 1.4 percent smaller.
Confusing watts with watt-hours. Watts are power; watt-hours are energy.
Ignoring efficiency. A motor draws more electrical power than it delivers.
Forgetting to multiply kilowatts by 1,000. Enter 150 kW as 150,000 W.
Comparing peak and continuous ratings. Many motors and amplifiers quote both a peak figure and a lower continuous one; compare like with like.
Assuming horsepower means the same everywhere. Check whether a figure is hp, PS or electrical horsepower before comparing two machines.