Power Unit Converter

This power unit converter changes a rate of energy use between 14 units, including watts, kilowatts, mechanical horsepower, metric horsepower (PS), BTU per hour and tons of refrigeration. The power unit converter uses this formula: 1 mechanical hp ≈ 745.699871582 W; 1 PS = 735.49875 W. All values go through the watt, one joule per second. Mechanical horsepower is about 745.70 W, metric horsepower is exactly 735.49875 W and electrical horsepower is exactly 746 W.

How to use the power unit converter

1. In the power unit converter, enter the power rating, for example 150 from an engine spec sheet.
2. Choose the unit it is in (hp) and the unit you want (kW).
3. Read the result; it is rounded only for display.

Check which horsepower a source means. US car specs use mechanical hp, while many European documents use PS, which is about 1.4 % smaller.

What is power?

Power is the rate of doing work or using energy. The SI unit is the watt (W), one joule per second. One mechanical horsepower is about 745.7 W, one metric horsepower about 735.5 W, and air conditioners are rated in BTU per hour or tons of refrigeration.

Conversion formula

The power unit converter stores every unit as a multiple of the base unit, the watt (W). To convert:

value in target unit = value × (factor of source unit ÷ factor of target unit)

where each factor is the size of the unit in watts.

Common power conversions

Power unit converter worked example

Convert 25 kilowatts to metric horsepower:

1. Factor of kW = 1,000 W
2. Factor of PS = 735.49875 W
3. 25 × 1,000 ÷ 735.49875 = 33.99054 PS

Power unit reference

Reverse factors for each power unit

The power unit converter reference table gives each unit in watts. To go the other way, divide by that factor; the list below does the division for you.

• Milliwatt (mW), SI: 1 mW = 0.001 W, so 1 W = 1,000 mW.
• Kilowatt (kW), SI: 1 kW = 1,000 W, so 1 W = 0.001 kW.
• Megawatt (MW), SI: 1 MW = 1,000,000 W, so 1 W = 1 × 10⁻⁶ MW.
• Gigawatt (GW), SI: 1 GW = 1 × 10⁹ W, so 1 W = 1 × 10⁻⁹ GW.
• Metric Horsepower (PS), Metric: 1 PS = 735.4987 W, so 1 W = 0.001359622 PS.
• Mechanical Horsepower (hp), US/Imperial: 1 hp = 745.6999 W, so 1 W = 0.001341022 hp.
• Electric Horsepower (hp electric), US: 1 hp electric = 746 W, so 1 W = 0.001340483 hp electric.
• BTU per Hour (BTU/h), US/Imperial: 1 BTU/h = 0.2930711 W, so 1 W = 3.412142 BTU/h.
• Foot-pound per Second (ft·lb/s), US/Imperial: 1 ft·lb/s = 1.355818 W, so 1 W = 0.7375621 ft·lb/s.
• Calorie per Second (thermochemical) (cal/s), Common: 1 cal/s = 4.184 W, so 1 W = 0.2390057 cal/s.
• Kilocalorie per Hour (kcal/h), Common: 1 kcal/h = 1.162222 W, so 1 W = 0.8604207 kcal/h.
• Erg per Second (erg/s), CGS: 1 erg/s = 1 × 10⁻⁷ W, so 1 W = 1 × 10⁷ erg/s.
• Ton of Refrigeration (TR), HVAC: 1 TR = 3,516.853 W, so 1 W = 0.0002843451 TR.

Engine power: horsepower and kilowatts

For hp to kW, multiply by 0.7457; for kW to hp, multiply by 1.341. EU registration documents list engine power in kW. Common ratings:

Watts to hp works the same way: divide watts by 745.7, so a 1,000 W motor is about 1.34 hp.

Appliance power in watts and BTU/h

Heating and cooling equipment in the US is often rated in BTU per hour. One watt is about 3.412 BTU/h, and one ton of refrigeration is 12,000 BTU/h:

A rating in watts is a rate. To find energy, multiply by time: a 1,500 W heater for 2 hours uses 3 kWh. The energy unit converter handles kWh, joules and BTU.

Related converters and calculators

For single conversions or engine and electrical calculations, try these:

• hp to kW: single-pair converter
• kW to hp: with a chart
• Horsepower calculator: from torque and rpm
• Engine horsepower calculator: from weight and speed
• Electricity calculator: running cost
• Energy unit converter: kWh, J and BTU
• Ohm’s law calculator: volts, amps and watts
• BTU calculator: room heating and cooling

Sources

The power unit converter is based on these sources: NIST Special Publication 811, Appendix B; BIPM: SI Brochure.

Questions about this calculator