Electric Horsepower to Watts: Exact Conversion

One electric horsepower (hp(E)) equals exactly 746 watts (W). To convert, multiply the horsepower value by 746; to convert watts back to electric horsepower, divide by 746.

Formula

The conversion is a simple linear relationship:

P(W) = P(hp(E)) × 746

• P(W) — power in watts
• P(hp(E)) — power in electric horsepower
• 746 — the exact conversion factor (watts per electric horsepower)

This is an exact definition, not an approximation. The electric horsepower is defined as exactly 746 watts, so no rounding is needed in the conversion itself.

Worked Example

Using the default value of 1 hp(E):

1. Start with the formula: P(W) = P(hp(E)) × 746
2. Plug in 1 hp(E): P(W) = 1 × 746
3. Result: P(W) = 746 W

So 1 electric horsepower equals exactly 746 watts. If you had a 2.5 hp(E) motor, the calculation would be 2.5 × 746 = 1,865 W.

Why There Are Different Horsepower Units

Horsepower was originally defined by James Watt in the late 18th century to compare the output of steam engines with the power of draft horses. Over time, different industries adopted slightly different definitions, leading to several variants:

• Mechanical horsepower (hp(I)) — 745.699872 W, used for most US and UK mechanical applications.
• Metric horsepower (PS or hp(M)) — 735.49875 W, common in Europe and Asia for car engines.
• Electric horsepower (hp(E)) — 746 W, used for electric motors and generators.
• Boiler horsepower — 9,809.5 W, used for steam boilers.

The electric horsepower was defined as exactly 746 watts to provide a clean, round number for electrical engineering calculations. The mechanical and metric versions are based on older physical measurements and therefore have more awkward decimal values.

When to Use Electric Horsepower vs. Watts

In practice, most modern electrical engineering uses watts or kilowatts (kW) directly. However, you may still encounter electric horsepower in:

• Nameplates of older electric motors, especially in North America
• HVAC equipment specifications (air conditioners, compressors)
• Industrial machinery documentation
• Electrical code tables and motor protection calculations

If you are sizing a circuit breaker or wire for a motor, you will typically need the full-load current, which depends on both power and voltage. The wattage alone does not determine current; you also need the voltage and power factor (for AC motors).

For example, a 1 hp(E) motor running on 120 V AC at unity power factor draws about 746 / 120 = 6.22 A. On 240 V, the same motor draws about 3.11 A. Real motors have a power factor below 1, so actual current will be higher.

Common Mistakes and Misconceptions

The most frequent error is using the wrong horsepower variant. If someone says “1 hp motor,� they usually mean mechanical horsepower (745.7 W), not electric (746 W). The difference is small — about 0.04% — but for precise engineering calculations it matters.

Another common mistake is confusing horsepower with horsepower-hours. Horsepower is a unit of power (rate of energy use), while horsepower-hours is a unit of energy (power × time). One horsepower-hour equals 746 watt-hours, or 0.746 kilowatt-hours.

Also, be careful with kilowatts: 1 hp(E) = 0.746 kW, not 0.75 kW. Rounding to 0.75 introduces a 0.5% error, which may be acceptable for rough estimates but not for billing or safety calculations.

Finally, the rated horsepower on a motor nameplate is the output mechanical power, not the electrical input power. Because motors are not 100% efficient, the electrical input will be higher. For example, a 1 hp(E) motor with 90% efficiency draws about 746 / 0.90 = 829 W from the supply.

Practical Conversion Examples

Here are some typical conversions to give you a feel for the scale:

• 0.5 hp(E) = 373 W
• 1 hp(E) = 746 W
• 2 hp(E) = 1,492 W
• 5 hp(E) = 3,730 W (3.73 kW)
• 10 hp(E) = 7,460 W (7.46 kW)

For a quick mental estimate, you can multiply horsepower by 750 to get approximate watts. This is within 0.5% of the exact value and is often sufficient for everyday conversations or rough sizing.

Limitations of the Conversion

The 746 W conversion factor applies only to electric horsepower. It does not apply to:

• Mechanical horsepower (use 745.7 W)
• Metric horsepower (use 735.5 W)
• Boiler horsepower (use 9,809.5 W)

Also, this conversion tells you the power rating of the motor, not the actual power it consumes at any moment. A motor’s actual power draw depends on its load, efficiency, and power factor. The nameplate rating is the maximum continuous output the motor is designed to deliver.

For AC circuits, the relationship between watts, volts, and amps is more complex than for DC. In AC, you must account for the power factor (cos φ):

P(W) = V × I × cos φ (for single-phase)

For three-phase, multiply by √3 (about 1.732).

Frequently Asked Questions

Is 1 hp exactly 746 watts?

Yes, for electric horsepower, 1 hp(E) is defined as exactly 746 watts. This is an exact definition, not a rounded value. However, mechanical horsepower is slightly different at 745.699872 W, so always check which unit you are using.

How do I convert watts to electric horsepower?

Divide the number of watts by 746. For example, 1,500 W ÷ 746 = 2.01 hp(E). For a rough estimate, divide by 750.

What is the difference between electric and mechanical horsepower?

Electric horsepower is defined as exactly 746 W, while mechanical horsepower is 745.699872 W. The difference is about 0.3 W per horsepower — negligible for most purposes but important for precise engineering work. Mechanical horsepower is used for engines and turbines, while electric horsepower is used for electric motors.

Why do motor nameplates show horsepower instead of watts?

Horsepower is a traditional unit that predates the widespread use of the watt. Many industries, especially in North America, still use horsepower for motor ratings because it gives a familiar sense of the motor’s capability. However, electrical calculations often require watts, so conversion is common.

Does the conversion change for three-phase motors?

No, the power conversion is the same: 1 hp(E) = 746 W regardless of the number of phases. However, when calculating current from power, the phase count matters because the formula includes √3 for three-phase systems. The power rating itself is phase-independent.

How many watts does a 2 hp motor use?

A 2 hp(E) motor has an output of 1,492 W. However, the electrical input will be higher due to inefficiency. If the motor is 85% efficient, it will draw about 1,492 / 0.85 = 1,755 W from the supply.

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in hp | Horsepower (electric) | hp(E) | 1 | 0
out watts | Watts | W | hp * 746 | 0