How to Convert HP to Watts for Motors (with Examples)

Horsepower is the standard measurement you will find on the nameplates of well pumps, air compressors, and table saws. However, when you need to size a generator, inverter, or solar battery bank, you must calculate the load in watts. The key to getting this right is knowing the critical difference between mechanical output watts and electrical input watts.

For full load planning and generator sizing, use the WattSizing Calculator.

Quick Answer

To convert mechanical horsepower to watts, the exact conversion is 1 HP = 746 watts. However, this only measures the mechanical output power. Because electric motors are not 100% efficient, they require more electrical power to run than they output mechanically. To find the actual electrical input watts you need to supply, use the formula: Input Watts = (HP × 746) ÷ Motor Efficiency.

Understanding the Scope: Output vs. Input

Before sizing any equipment, you must understand what horsepower actually measures on an electric motor.

Mechanical Output Power: This is the physical work the motor performs (e.g., spinning a saw blade or pumping water). By definition, 1 Horsepower equals exactly 746 Watts of mechanical work.
Electrical Input Power: This is the electricity the motor pulls from your wall outlet or generator to do that work. Because energy is lost to heat and friction inside the motor, the input watts will always be higher than the output watts.
Motor Efficiency: A percentage indicating how well the motor converts electrical input into mechanical output. A standard AC motor usually operates between 80% and 90% efficiency.

This guide covers how to calculate the electrical input watts required to run a motor. It does not cover calculating the specific wire gauge or breaker size, which requires consulting the National Electrical Code (NEC) based on full-load amperage.

Crucial Details Often Overlooked

When converting HP to watts for backup power planning, many people make dangerous underestimations. Keep these realities in mind:

The 746W Myth: Many basic guides tell you to simply multiply HP by 746 to size a generator. If you do this, you are only calculating the mechanical output. You will drastically underestimate the electrical load, leading to tripped breakers or stalled generators.
Starting Surge (LRA): The HP conversion formulas only give you the running watts. When an electric motor first starts, it draws a massive surge of current (Locked Rotor Amps) that can be 3 to 5 times higher than its running watts. Your power source must be able to handle this brief spike.
Power Factor: While efficiency accounts for heat loss, AC motors also have a power factor (often around 0.8). This means the Apparent Power (Volt-Amps) drawn from a generator will be even higher than the Real Power (Watts) calculated here.

Conversion Equations

1) Mechanical Output Power

Use this if you only need to know the physical work being done. Output Watts = HP × 746

2) Estimated Electrical Input Power

Use this to determine how much power your generator or inverter must supply. Input Watts = (HP × 746) ÷ Motor Efficiency

Note: If the motor's nameplate does not list an efficiency rating, it is generally safe to assume an efficiency of 0.80 (80%) for small motors and 0.85 (85%) for motors over 1 HP.

Illustrative Worked Examples

Here are transparent calculations showing how these formulas apply in the real world. Note: These are illustrative examples; always check your specific equipment's nameplate.

Example 1: A 1 HP Well Pump

You have a 1 HP submersible well pump with an efficiency rating of 85% (0.85).

Output: 1 HP × 746 = 746 Watts (Mechanical work)
Input Calculation: 746 Watts ÷ 0.85 = 877.6 Watts
Result: The generator must supply approximately 878 running watts to operate this 1 HP pump.

Example 2: A 3 HP Air Compressor

You want to run a heavy-duty 3 HP compressor with an assumed efficiency of 82% (0.82).

Output: 3 HP × 746 = 2,238 Watts (Mechanical work)
Input Calculation: 2,238 Watts ÷ 0.82 = 2,729 Watts
Result: You must supply roughly 2,730 running watts. If you only multiplied by 746, you would have underestimated the load by nearly 500 watts.

HP to Watts Reference Table

Here is a quick reference table showing the difference between mechanical output and realistic electrical input (assuming an average 85% efficiency).

Motor Size	Mechanical Output (W)	Estimated Electrical Input (W)
0.5 HP	373 W	439 W
0.75 HP	560 W	659 W
1 HP	746 W	878 W
1.5 HP	1,119 W	1,316 W
2 HP	1,492 W	1,755 W
3 HP	2,238 W	2,633 W

Practical Checklist: Sizing a Generator for a Motor

If you are converting HP to watts to buy a generator, follow these steps:

Calculate Running Watts: Use the input formula (HP × 746) ÷ Efficiency to find the continuous electrical load.
Estimate Starting Watts: Multiply your running watts by 3 to estimate the starting surge required to get the motor spinning.
Add Concurrent Loads: Add the running watts of any other devices (lights, chargers) that will run at the same time.
Select the Generator: Choose a generator where the Starting/Surge Watts rating exceeds your estimated starting watts, and the Running Watts rating exceeds your total continuous load.

FAQs

Is 1 HP always exactly 746 W in real systems?

Mechanically, yes. 1 HP is defined as 746 watts of physical work. However, electrically, a 1 HP motor will always consume more than 746 watts of electricity due to internal inefficiencies and heat loss.

Can I size a generator from HP alone?

Only as a very rough estimate. Best practice is to ignore the HP rating and instead look at the nameplate for the Volts and Amps. Multiplying Volts × Amps will give you a much more accurate picture of the electrical load.

Do I need power factor in HP-to-watts conversion?

You do not need the power factor to convert HP to Real Power (Watts). However, you do need the power factor if you want to calculate the Apparent Power (Volt-Amps or VA), which is how many generators and inverters are rated.

Why are my measured watts lower than the formula?

Your motor may not be fully loaded. The HP rating on the nameplate is the maximum mechanical output. If a 2 HP motor is only doing 1 HP worth of work (like a saw cutting through very thin wood), it will draw significantly fewer watts than the formula predicts.

What is the difference between Peak HP and Continuous HP?

Continuous HP is the power the motor can sustain all day without overheating. Peak HP (often advertised on shop vacuums and air compressors) is a theoretical maximum the motor hits for a fraction of a second before stalling. Always base your electrical calculations on the Continuous HP rating.

How do I find motor efficiency if it's not on the label?

If the nameplate does not list efficiency (sometimes labeled as "EFF"), you can safely estimate 80% (0.80) for motors under 1 HP, and 85% (0.85) for motors between 1 HP and 3 HP.

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How to Convert HP to Watts for Motors