Everything routes through the watt
The watt is the SI base for power (1 W = 1 joule per second). Every other unit is defined against it, so a conversion is just two steps: turn the input into watts, then divide by the target unit's watt value.
Watt values of the common units
| Unit | In watts | Where you meet it |
|---|---|---|
| 1 kW | 1,000 W | Appliances, heaters, EU motors |
| 1 MW | 1,000,000 W | Power plants, grid |
| 1 hp (mechanical) | 745.7 W | US engines |
| 1 hp (metric, PS/KM) | 735.5 W | EU car engines |
| 1 hp (electrical) | 746 W | US electric motors |
| 1 BTU/h | 0.2931 W | Heating, air conditioning |
| 1 ton refrigeration | 3,516.85 W | US AC capacity |
The catch: three different horsepowers
"Horsepower" is not one number. The US mechanical hp, European metric hp and electrical hp sit about 1.4% apart, which is why a car's power reads slightly differently on each side of the Atlantic.
Metric horsepower is about 1.4% smaller than mechanical, so 100 metric hp is only ~98.6 US hp.
Handy shortcuts
- kW to BTU/h. Multiply by 3,412. A 3.5 kW heater is about 12,000 BTU/h.
- hp to kW. Multiply by 0.746 (mechanical). A 150 hp engine is about 112 kW.
- Cooling tons. One ton = 12,000 BTU/h = 3.5 kW. A 2-ton AC unit removes 24,000 BTU/h.
Common questions
How many watts are in one horsepower?
Mechanical horsepower, the US standard, is 745.7 watts. Metric horsepower (PS/KM, used across Europe) is 735.5 watts, and electrical horsepower is exactly 746 watts. They differ by about 1.4 percent, so which one is meant matters for engine specs.
How do I convert watts to BTU per hour?
Multiply watts by 3.412. So 1 kW is 3,412 BTU/h, and a 12,000 BTU/h air conditioner (one ton of cooling) is about 3.5 kW.
What is a ton of refrigeration in watts?
One ton of cooling is 12,000 BTU/h, which is 3,516.85 watts, or about 3.5 kW. It comes from the rate needed to melt one short ton of ice in 24 hours.
