Why are there different types of horsepower?

The three definitions arose from different measurement traditions. Mechanical HP came from James Watt's 1783 experiments with mill horses and uses imperial units (foot-pounds per minute). Metric HP was created when the metric system needed an equivalent, using kilograms and meters instead. Electrical HP was rounded to exactly 746W for convenience in power engineering calculations. Each stuck in its respective industry because changing standards is harder than living with three slightly different numbers.

Is 1 horsepower exactly 746 watts?

Only if you mean electrical horsepower, which is defined as exactly 746W. Mechanical (imperial) horsepower is 745.69987 watts — usually rounded to 745.7W. Metric horsepower is 735.49875W, rounded to 735.5W. The difference between mechanical and electrical HP is just 0.3W, which is irrelevant for practical purposes. The difference between mechanical and metric HP is about 10.2W per HP, which adds up on larger motors — a "100 HP" motor is 74,570W mechanical but only 73,550W metric.

When should I use metric horsepower instead of mechanical?

Use metric HP (PS/CV/pk) when working with European or Asian equipment that was rated using the metric system. Vehicle specifications from Germany, Japan, and most of the EU use metric horsepower. If you are importing a motor or pump rated in kW from a European manufacturer, the metric HP conversion gives a more accurate comparison to a US-market motor than the mechanical HP conversion. When in doubt, convert everything to watts or kilowatts — that eliminates the ambiguity entirely.

How do I convert motor HP to the breaker size I need?

First, convert HP to watts using this calculator. Then divide watts by (voltage x efficiency x power factor) to get full load amps (FLA). Finally, size your breaker per NEC Table 430.52 — typically 250% of FLA for an inverse-time breaker. A 5 HP motor at 230V single-phase with 85% efficiency and 0.85 PF draws about 22.5A, requiring a 60A breaker. Our motor FLA calculator does this full calculation in one step, including the NEC breaker multiplier. For long motor circuit runs, check the voltage drop calculator to ensure the motor receives adequate voltage.

HP to Watts Calculator — Instant

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Convert between horsepower and watts in either direction. Select mechanical, electrical, or metric horsepower for the correct conversion factor — the difference matters when sizing circuits, comparing motor ratings across countries, or checking manufacturer specs.

Where Horsepower Came From

James Watt coined the term "horsepower" around 1783 to market his improved steam engine. He needed a way to tell coal mine operators how many horses his engine could replace. After observing mill horses, he estimated that a strong horse could do 33,000 foot-pounds of work per minute — turning a mill wheel by walking in a circle. That figure became the definition of one mechanical horsepower.

Whether Watt's measurement was accurate is debatable. Modern studies suggest a horse can sustain about 0.7 HP over an extended period, with brief bursts up to 15 HP. But the number stuck because it was useful, not because it was precise. Two centuries later, we still rate motors, engines, and compressors in a unit based on an 18th-century estimate of equine output.

The watt — named after the same James Watt — is the SI unit of power. One watt equals one joule per second. It is a precisely defined unit, unlike horsepower, which has accumulated three different definitions depending on the industry and country using it. When you see a motor rated in HP, the first question should be: which horsepower? For most US-market motors, it is mechanical HP. Our motor FLA calculator uses this definition when converting HP to the current the motor draws.

Three Types of Horsepower Compared

Mechanical (Imperial) Horsepower — 745.7 watts. This is the original Watt definition: 33,000 foot-pounds per minute. Used on virtually all US motor nameplates, compressors, pumps, and power tools. When an American spec sheet says "5 HP" without qualification, it means mechanical horsepower. This is the default you should use for wire sizing, breaker selection, and load calculations in North American electrical work.

Electrical Horsepower — 746 watts. Defined by convention as exactly 746 watts. The difference from mechanical HP (0.3W) is trivial in practice, but it exists because electrical engineers wanted a round number for calculations. You will encounter this in generator ratings, UPS specifications, and some industrial motor documentation. For all practical circuit sizing purposes, 745.7W and 746W are interchangeable.

Metric Horsepower (PS) — 735.5 watts. Defined as 75 kilogram-force meters per second. Common in European and Asian automotive specifications. A car rated at 200 PS (Pferdestärke, literally "horse strength") has about 197 mechanical HP. The 1.4% difference occasionally causes confusion when comparing vehicle specs across markets. If you are converting a European motor rating for use in a North American installation, the metric-to-watts conversion gives you the correct power figure to work with.

HP to Watts Conversion Table (Mechanical HP)

Horsepower	Watts	Kilowatts	Common Application
0.5 HP	373	0.37	Bathroom exhaust fan, small sump pump
1 HP	746	0.75	Garbage disposal, garage door opener
2 HP	1,491	1.49	Pool pump, dust collector
3 HP	2,237	2.24	Table saw, commercial mixer
5 HP	3,729	3.73	Air compressor, large shop tool
7.5 HP	5,593	5.59	HVAC blower, small irrigation pump
10 HP	7,457	7.46	Industrial fan, grain auger
15 HP	11,186	11.19	Large compressor, elevator motor
20 HP	14,914	14.91	Commercial HVAC, conveyor system
25 HP	18,643	18.64	Large pump station, industrial blower
50 HP	37,285	37.29	Industrial motor, large chiller
100 HP	74,570	74.57	Heavy industrial, large generator

Values use the mechanical horsepower definition (745.7 watts). For metric HP, multiply the HP value by 735.5 instead. The kilowatt column is what you will find on European motor nameplates — a "7.5 kW" motor is the metric equivalent of a 10 HP motor.

Worked Examples

Sizing a Circuit for a 5 HP Workshop Motor

Context

You purchased a 5 HP air compressor for your workshop. The nameplate says 5 HP mechanical. You need to know the wattage to figure out what circuit and breaker the compressor requires on a 230V single-phase supply.

Calculation

5 HP x 745.7 W/HP = 3,728.5 W

This is the mechanical output power. The electrical input power is higher because no motor is 100% efficient.

At 85% efficiency: 3,728.5 / 0.85 = 4,386 W input

Current at 230V: 4,386 / (230 x 0.85 PF) = 22.4 A

Interpretation

The motor draws about 22.4A at full load. Per NEC, conductors must handle 125% of motor FLA (28A minimum), and a typical inverse-time breaker would be sized at 250% of FLA (56A, rounded to a standard 60A breaker).

Takeaway

HP is just the starting point — you need FLA for the actual circuit design. Our motor FLA calculator handles the full conversion from HP to amps with efficiency and power factor included, and shows the NEC breaker sizing.

Converting a European Motor Rating to US Standards

Context

You are importing a German-made pump rated at 3 PS (metric horsepower). You need to know the equivalent wattage and how it compares to a US 3 HP pump for your 240V installation.

Calculation

Metric HP: 3 PS x 735.5 W/PS = 2,206.5 W

US mechanical HP: 3 HP x 745.7 W/HP = 2,237.1 W

Difference: 2,237.1 - 2,206.5 = 30.6 W (1.4% less power from the metric-rated pump)

Interpretation

The German pump delivers about 31W less than a US-rated 3 HP pump. In most applications, this 1.4% difference is insignificant — the pump will perform essentially the same. However, the circuit sizing should use the actual wattage (2,207W) for accurate current calculations.

Takeaway

When cross-referencing international motor specs, always convert to watts first to avoid apples-to-oranges comparisons. To find the actual current this pump draws on your 240V supply, enter the wattage in our amps draw calculator.

Frequently Asked Questions

Glossary

Mechanical Horsepower

The original horsepower definition from James Watt, equal to 33,000 foot-pounds per minute or 745.7 watts. This is the standard used on virtually all US motor nameplates, power tools, compressors, and pumps. When a product says "HP" without further qualification in North America, it means mechanical horsepower.

Electrical Horsepower

A horsepower definition set at exactly 746 watts by convention for use in electrical power calculations. The 0.3W difference from mechanical HP is negligible in practice. You may encounter this in generator output ratings, UPS specs, and some industrial electrical documentation.

Metric Horsepower (PS)

The metric system equivalent of horsepower, equal to 75 kilogram-force meters per second or 735.5 watts. Known as PS (Pferdestärke) in German, CV (cheval-vapeur) in French, and pk (paardenkracht) in Dutch. Standard in European and Asian automotive and industrial specifications. About 1.4% less than mechanical HP.

Want to know what that motor costs to run? Our kWh calculator turns watts and run hours into daily, monthly, and yearly electricity costs. Try it now →

The confusion between horsepower types rarely causes safety problems — the differences are small. But it can cause purchasing mistakes. A European-market motor rated at 10 PS delivers about 1.4% less power than a US motor rated at 10 HP. On a small motor that is negligible. On a 200 HP industrial drive, 1.4% is nearly 3 HP of "missing" capacity. When comparing equipment across markets, convert everything to watts first. Watts are watts everywhere — no regional variants, no historical baggage, no ambiguity.

Last updated: March 27, 2026

Written and maintained by Dan Dadovic, Developer & Off-Grid Energy Enthusiast. On the energy side, Dan has hands-on experience with residential solar panel installation, DIY battery bank construction, off-grid power systems, and wind power — all from building and maintaining his own systems..

Disclaimer: Calculator results are estimates based on theoretical formulas. Actual performance varies with temperature, battery age, load patterns, and equipment condition. For critical electrical work, consult a licensed electrician.