Watt and horsepower
Units of power.
A bulb and an engine
A bulb is labelled in watts. A car engine is often described in horsepower.
How do you think the watt is defined?
What this lesson covers
The idea
The SI unit of power is the watt (W), equal to 1 joule of work done per second (1 W = 1 J s⁻¹); power is also expressed in horsepower (hp), where 1 hp = 746 W.
A bulb and an engine
A bulb is labelled in watts. A car engine is often described in horsepower.
How do you think the watt is defined?
- As 1 joule of work done per second
- As 1 newton of force
- As 1 metre of distance
Two units
Open each tab.
Two ways to say power
One watt is one joule of work each second. Horsepower is an older unit, and 1 hp is 746 W, so a power given in horsepower is changed to watts by multiplying by 746.
The SI unit of power is the watt (W), equal to 1 joule of work done per second: 1 W = 1 J s⁻¹. Power is also expressed in horsepower (hp), where 1 hp = 746 W.
Notes
1 W = 1 J s⁻¹, the SI unit of power. 1 hp = 746 W.
Check yourself
1 watt is…
A machine does 600 J of work in 3 s. What is its power, in W?
Answer: 200 W
P = 600 J ÷ 3 s = 200 J s⁻¹ = 200 W.
An engine has a power of 2 hp. Using 1 hp = 746 W, what is its power in W?
Answer: 1492 W
2 × 746 W = 1492 W.
Why is the unit horsepower named like this?
- 1 joule of work done per second — correct. Yes!
- 1 joule of work done in 1 minute. The watt is per second: 1 W = 1 J s⁻¹.
- 1 newton of force acting for 1 second. The watt measures work done per second, in joules per second.
- Horses are measured in watts. The name comes from comparing the powers of early engines with the power of horses.
- Engine powers were compared to the power of actual horses — correct. Yes!
- 1 hp is the weight of a horse. The unit compares engine power with the power of horses. It is not a weight.