Power
W, kW, horsepower, BTU/hr
About This Calculator
Power is the rate of energy transfer or work done per unit time. The watt (SI unit) is used for electrical appliances; kilowatts for larger systems; horsepower for engines and motors; and BTU/hr for HVAC heating and cooling capacity. Converting between them is essential for engineering, energy planning, and comparing equipment specifications.
Formula
1 kilowatt (kW) = 1,000 watts (W)
1 horsepower (hp) = 745.7 watts = 0.7457 kW
1 BTU/hour = 0.29307 W
1 ton of refrigeration = 3,517 W = 12,000 BTU/hr
Example Calculation
Convert a 150 hp car engine to kilowatts and watts.
- kW = 150 × 0.7457 = 111.9 kW
- W = 111.9 × 1,000 = 111,855 W
150 hp = 111.9 kW = 111,855 W
Power Unit Conversions
| Unit | Watts (W) | kW | hp | BTU/hr |
|---|---|---|---|---|
| 1 W | 1 | 0.001 | 0.001341 | 3.412 |
| 1 kW | 1,000 | 1 | 1.341 | 3,412 |
| 1 hp | 745.7 | 0.7457 | 1 | 2,545 |
| 1 BTU/hr | 0.2931 | 0.000293 | 0.000393 | 1 |
| 1 ton (refrig.) | 3,517 | 3.517 | 4.716 | 12,000 |
Frequently Asked Questions
What is the difference between mechanical and electrical horsepower?
Mechanical (or imperial) horsepower = 745.7 W, defined as the power to lift 550 lb by 1 ft in 1 second. Metric horsepower (PS, Pferdestärke) = 735.5 W. Electrical horsepower = 746 W exactly. European car specs often use PS; US specs use hp. The difference is about 1.4%.
How is BTU/hr used for air conditioners?
Air conditioner cooling capacity is rated in BTU/hr (in the US). A 12,000 BTU/hr unit is called '1 ton' of cooling (from the era of ice cooling). Typical room air conditioners are 5,000-24,000 BTU/hr; central systems are 24,000-60,000+ BTU/hr.
How many watts does a typical household use?
Average US home power consumption is about 1.2 kW continuously (29 kWh/day). Peak demand is higher — air conditioners alone can draw 3-5 kW. Your electricity bill is in kWh (kilowatt-hours), which is power × time.
What is the difference between power and energy?
Power is the rate of energy use (watts = joules per second); energy is the total amount used (watt-hours or joules). A 100 W light bulb running for 10 hours uses 1,000 Wh = 1 kWh of energy. Your electricity bill charges for energy (kWh), not power (kW).