Power to Watts Calculator: Convert HP, kW & More

This calculator converts power measurements between watts, kilowatts, horsepower (mechanical and metric), and other common units. Enter a value and select its unit, then choose the unit you want to convert to; the result appears instantly. The calculator uses the standard conversion factors for each unit, so you can trust the numbers for engineering, physics, or everyday appliance ratings.

Formula

The conversion uses a simple ratio: Result = (Input Value × From Unit Factor) / To Unit Factor.

Each unit has a fixed factor representing how many watts it equals:

• Watt (W) = 1
• Kilowatt (kW) = 1000 W
• Megawatt (MW) = 1,000,000 W
• Mechanical horsepower (hp(I)) = 745.699872 W
• Metric horsepower (hp(M)) = 735.49875 W
• BTU per hour (BTU/hr) = 0.29307107 W
• Foot-pound per second (ft·lb/s) = 1.35581795 W

All units are defined relative to the watt, the SI unit of power. The watt itself is one joule per second, or in electrical terms, volts × amperes.

Worked Example

Using the default values: convert 100 watts to kilowatts.

1. Input value = 100
2. From unit factor (W) = 1
3. To unit factor (kW) = 1000
4. Result = (100 × 1) / 1000 = 0.1 kW

The calculator shows 0.1 kW (rounded to 4 decimals). This matches the manual calculation.

When to Use This Calculator

You might need to convert power units when:

• Comparing the power of car engines (often given in horsepower) with electric motors (given in kW).
• Reading an appliance label that lists watts but you need kilowatt-hours for energy cost estimates.
• Working on physics homework or engineering projects that require consistent SI units.
• Understanding the output of heaters or air conditioners, which sometimes list BTU/hr instead of watts.

Always check which horsepower you are dealing with: mechanical (imperial) vs. metric. They differ by about 1.4%. The calculator includes both, so you can see the difference.

How to Read the Result

The result is the equivalent power in the target unit. For example, converting 1 mechanical horsepower to watts gives 745.7 W, which is roughly the power of a small space heater. Converting 1 kW to BTU/hr yields about 3412 BTU/hr, a common conversion for heating and cooling equipment.

If you convert a small value to a large unit (e.g., 1 watt to megawatts), the result will be a small decimal (0.000001 MW). The calculator rounds to 4 decimal places, so very small conversions may show 0.0000. In that case, convert to a smaller unit instead (e.g., from W to kW).

Common Mistakes and Misconceptions

• Mixing up horsepower types: Using mechanical horsepower when the spec says metric horsepower (or vice versa) leads to a 1.4% error—small but sometimes important.
• Confusing power and energy: Watts measure power (rate of energy use), while watt-hours or joules measure energy. A 100 W bulb running for 10 hours uses 1000 Wh = 1 kWh, not 1000 W.
• Ignoring units in formulas: When calculating electrical power (P = V × I), if voltage is in volts and current in amperes, the result is in watts. If you use millivolts or milliamps, you must convert first.
• Rounding too early: If you round intermediate conversions, the final result may be off. Do the conversion in one step, as the calculator does.

Limits of the Formula

This conversion is purely multiplicative, so it has no physical limits—any positive number works. However, the accuracy depends on the unit definitions. The factors used are internationally accepted exact values (for example, 1 hp(I) = 745.699872 W exactly). For everyday purposes, you can round to 746 W and 735 W for metric, but for precise engineering, use the full factors.

The calculator does not account for efficiency. For example, a motor’s input electrical power (watts) and its output mechanical power (horsepower) are not directly convertible without knowing the motor’s efficiency. This calculator converts units, not energy losses.

Alternatives and Related Conversions

If you need to convert power in other contexts:

• dBm to watts: Used in telecommunications; dBm is a logarithmic power ratio relative to 1 milliwatt.
• VA to watts: For AC circuits, apparent power (VA) times power factor gives real power (W).
• Joules to watts: If you know energy in joules and time in seconds, divide to get watts.
• Calories per second to watts: Common in nutrition and physics (1 cal/s ≈ 4.1868 W).

Frequently Asked Questions

How many watts are in a horsepower?

There are two common definitions: mechanical horsepower equals 745.699872 watts, and metric horsepower equals 735.49875 watts. The mechanical one is used in the US and UK for engines; the metric one is used in Europe and Asia. Always check which one your spec uses.

What is the difference between kW and kWh?

Kilowatts (kW) measure power—the rate of energy use at a given instant. Kilowatt-hours (kWh) measure energy—the total amount consumed over time. A 2 kW heater running for 3 hours uses 6 kWh of energy. Your electricity bill charges for kWh, not kW.

How do I convert BTU/hr to watts?

Multiply the BTU/hr value by 0.29307107. For example, a 10,000 BTU/hr air conditioner is about 2,931 watts. This conversion is common when comparing heating and cooling equipment from different regions.

Why are there two different horsepower units?

James Watt originally defined horsepower based on a horse’s ability to lift a weight. The mechanical (imperial) horsepower is based on 550 ft·lb/s. The metric horsepower was later defined as 75 kgf·m/s (735.49875 W). They are close but not identical, leading to the two standards.

Can I convert watts to amps?

Only if you also know the voltage. For DC circuits, amps = watts / volts. For AC, you also need the power factor: amps = watts / (volts × power factor). This calculator does not do that conversion because it requires additional information.

Is a higher wattage always better?

Not necessarily. Higher wattage means more power consumption, which may mean more heat or more work done, but it also means higher running costs. For appliances like light bulbs, higher wattage means brighter light (for the same technology), but LED bulbs produce the same light with far fewer watts than incandescent bulbs.

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in value | Power value | - | 100 | 0
sel from_unit | From unit | W=1, kW=1000, MW=1000000, hp (mech)=745.699872, hp (metric)=735.49875, BTU/hr=0.29307107, ft·lb/s=1.35581795
sel to_unit | To unit | W=1, kW=1000, MW=1000000, hp (mech)=745.699872, hp (metric)=735.49875, BTU/hr=0.29307107, ft·lb/s=1.35581795
out result | Converted value | - | value * from_unit / to_unit | 4