HP to Watts Converter: Formula & Examples

This calculator converts horsepower (hp) to watts (W) using the standard mechanical horsepower definition, where 1 hp equals exactly 745.699872 W. You enter the power value in horsepower, and the calculator returns the equivalent power in watts, along with the result in kilowatts for convenience. The conversion is a simple multiplication, but the calculator removes any chance of arithmetic error.

Formula

The conversion from horsepower to watts uses the fixed relationship:

P(W) = P(hp) × 745.699872

Where:

• P(W) is the power in watts (W)
• P(hp) is the power in mechanical horsepower (hp)
• 745.699872 is the exact number of watts in one mechanical horsepower

For kilowatts, divide the watt result by 1000: P(kW) = P(hp) × 0.745699872

Worked Example

Using the default value of 1 hp:

1. Start with the formula: P(W) = P(hp) × 745.699872
2. Substitute 1 hp: P(W) = 1 × 745.699872
3. Multiply: P(W) = 745.699872 W
4. Round to two decimal places: 745.70 W

For kilowatts: 745.699872 ÷ 1000 = 0.745699872 kW, rounded to three decimals gives 0.746 kW. This matches the calculator output.

Why Horsepower Exists and What It Means

Horsepower was defined in the late 18th century by James Watt to compare the output of steam engines with the work a horse could do. He estimated that a horse could lift 33,000 pound-feet per minute, which became the mechanical horsepower unit. That value translates to exactly 745.699872 watts in the modern metric system.

Today, horsepower is still used to rate engines in cars, lawn mowers, generators, and industrial motors, while watts are the standard unit in electrical engineering and most of the world. Because the two units measure the same physical quantity—power, or the rate of doing work—converting between them is a direct multiplication.

Different Types of Horsepower

Not all horsepower is the same. The conversion factor depends on which definition you are using:

• Mechanical horsepower (hp): 1 hp = 745.699872 W. This is the most common in the US and is used for engines and motors.
• Metric horsepower (PS): 1 PS = 735.49875 W. Used in Europe and Japan for car engines; it is slightly smaller than mechanical horsepower.
• Electrical horsepower: 1 hp = 746 W exactly. Used for electric motors in some contexts, though the mechanical value is more common.
• Boiler horsepower: 1 boiler hp = 9,809.5 W. Used for steam boilers, a much larger unit.

If you are converting an engine rating, check whether the original figure is mechanical or metric horsepower. The difference is about 1.4%, which can be significant for large engines.

How to Convert Manually

To convert horsepower to watts by hand, multiply the horsepower value by 745.699872. For example:

• 5 hp × 745.699872 = 3,728.49936 W (about 3.73 kW)
• 0.5 hp × 745.699872 = 372.849936 W (about 0.373 kW)
• 150 hp × 745.699872 = 111,854.9808 W (about 111.85 kW)

For quick estimates, you can use 746 W per hp, which is accurate to about 0.04%. That is fine for most everyday purposes, but use the exact factor when precision matters, such as in engineering calculations or when sizing electrical components.

When You Need This Conversion

You might need to convert horsepower to watts in several real situations:

• Electrical load calculations: A generator or inverter rated in watts must be larger than the total wattage of the devices it powers. If a motor is rated in hp, convert it to watts to compare with the generator’s output.
• Comparing engines: Car specs in different countries may use hp or kW. Converting to watts (or kW) lets you compare directly.
• Physics and engineering homework: Many problems require consistent units, and watts are the SI unit for power.
• Understanding energy bills: While electricity is billed in kilowatt-hours, knowing the wattage of a motor helps estimate its running cost.

Remember that the watt rating of an electric motor is the mechanical output power, not the electrical input power. Real motors are not 100% efficient, so an electric motor rated 1 hp (746 W output) will draw more than 746 W from the wall, typically 800–900 W depending on efficiency.

Common Mistakes and Misconceptions

A frequent error is using the wrong horsepower definition. If a European car lists power in PS (metric horsepower), multiplying by 745.7 will overstate the wattage by about 1.4%. Always confirm whether the figure is hp or PS.

Another mistake is confusing horsepower with kilowatts directly. Some assume 1 hp = 1 kW, but that is off by about 25%. The correct relationship is roughly 1 hp = 0.746 kW, so a 100 hp engine is about 74.6 kW, not 100 kW.

Also, do not confuse power with energy. Horsepower and watts measure power (rate of energy use), while watt-hours and kilowatt-hours measure energy. A 1 hp motor running for one hour consumes about 0.746 kWh of energy (if 100% efficient), but the motor’s power rating is still 746 W regardless of how long it runs.

Limitations of the Conversion

The conversion is exact and universal for mechanical horsepower. However, the practical limitations come from the source of the horsepower figure:

• Engine power ratings are measured under specific conditions (SAE net, DIN, etc.) and may vary slightly between tests.
• Electric motor nameplate ratings are output power, not input power. The actual electrical draw is higher due to inefficiency.
• For vehicles, the power at the wheels is lower than the engine’s rated power due to drivetrain losses, typically 10–20% less.

For most conversions, the mathematical factor is all you need. The uncertainty lies in the original measurement, not in the conversion itself.

Frequently Asked Questions

How many watts is 1 horsepower?

One mechanical horsepower equals exactly 745.699872 watts. For most practical purposes, you can round to 746 watts.

Is 1 hp equal to 1 kW?

No. 1 hp is approximately 0.746 kW. To get 1 kW, you need about 1.34 hp. The two units are close but not equal.

How do I convert kW to hp?

Divide the kilowatt value by 0.745699872 (or multiply by 1.34102). For example, 10 kW ÷ 0.7457 = 13.41 hp.

What is the difference between hp and PS?

PS (Pferdestärke) is metric horsepower, equal to 735.49875 watts. It is about 1.4% smaller than mechanical horsepower. A car rated 100 PS is about 98.6 hp.

Why do electric motors list both hp and watts?

Electric motors in the US often list horsepower for tradition, while watts are the SI unit. The watt rating on a motor is the mechanical output power. The electrical input power is higher due to losses.

Can I use the calculator for metric horsepower?

No, this calculator uses mechanical horsepower. For metric horsepower, multiply by 735.49875 instead of 745.699872.

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in hp | Horsepower | hp | 1 | 0
out watts | Watts | W | hp * 745.699872 | 2
out kilowatts | Kilowatts | kW | hp * 0.745699872 | 3