Last updated: 2026-05-18

Supported Units

Power — the rate at which energy is transferred or converted — is measured in several unit families. This converter covers metric units (mW, W, kW, MW), mechanical power (hp, PS), and thermal power (BTU/h, cal/s, kcal/h). Enter a value in any field and all others update automatically.

Key Conversion Factors

Unit	Watts (W)
1 milliwatt (mW)	0.001 W
1 watt (W)	1 W
1 kilowatt (kW)	1,000 W
1 megawatt (MW)	1,000,000 W
1 mechanical horsepower (hp)	745.69987 W
1 metric horsepower / PS	735.49875 W
1 BTU/h	0.29307 W
1 cal/s	4.184 W
1 kcal/h	1.163 W

Mechanical vs. Metric Horsepower

There are two common forms of horsepower in everyday use:

• Mechanical hp (used in the US): defined as exactly 33,000 foot-pounds per minute, equal to 745.69987 W.
• Metric PS (Pferdestärke, used in Europe and Japan): defined as the power needed to lift 75 kg by 1 meter in 1 second, equal to 735.49875 W.

The two differ by about 1.4%. A European car rated at 100 PS produces approximately 98.6 mechanical hp. When comparing vehicle specs across US and European sources, check which unit is being used.

Thermal Power Units

BTU/h (British Thermal Units per hour) is the dominant unit for heating and cooling equipment in North America. Air conditioner capacities are routinely stated in BTU/h:

BTU/h	kW (approx.)	Typical use
5,000	1.46	Small room A/C
12,000	3.52	"1-ton" A/C unit
24,000	7.03	Large room / small house
36,000	10.55	Medium house

The thermal calorie (cal/s) and kilocalorie per hour (kcal/h) appear in laboratory and food-energy contexts. Note: 1 cal/s = 4.184 W (thermochemical calorie).

Practical Notes

• Household appliances typically range from a few watts (LED bulb: 8–15 W) to a few kilowatts (electric kettle: 2–3 kW, oven: 2–4 kW).
• Electric vehicle motors are often rated in kW; a 100 kW motor produces about 134 hp.
• Power plant outputs are in megawatts (MW) or gigawatts (GW). A typical nuclear reactor unit produces around 1,000 MW = 1 GW.

Frequently Asked Questions (FAQ)

How do I convert watts to horsepower?

One mechanical horsepower (hp) equals exactly 745.69987 watts. To convert watts to horsepower, divide by 745.69987. For example, a 1,000 W motor produces about 1.341 hp. In everyday use this figure is often rounded to 746 W per hp, but the precise value is 745.69987 W.

What is the difference between mechanical hp and metric HP (PS)?

Mechanical horsepower (hp) is used in the United States and equals 745.69987 W. Metric horsepower, known as PS (from the German Pferdestärke, "horse strength"), is used in Europe and Japan and equals 735.49875 W — about 1.4% less than the mechanical hp. When comparing car engine outputs between US and European spec sheets, make sure you are using the same unit: a "100 PS" European engine is about 98.6 hp in US terms.

What is BTU/h and when is it used?

BTU per hour (BTU/h) is a unit of power widely used in HVAC (heating, ventilation, and air conditioning). One BTU/h equals approximately 0.2931 W. Air conditioner cooling capacities are commonly rated in BTU/h — a 12,000 BTU/h unit (often called a "1-ton" unit) delivers about 3,516 W of cooling. To convert BTU/h to watts, multiply by 0.29307.