Power

Watts, kilowatts, metric horsepower, mechanical horsepower, kcal/h, BTU/h and tons of refrigeration.

Value

Unit

Equivalents

watt	—	W
kilowatt	—	kW
megawatt	—	MW
metric horsepower (PS)	—	CV
mechanical horsepower	—	hp
kilocalorie per hour	—	kcal/h
BTU per hour	—	BTU/h
kilogram-force metre per second	—	kgf·m/s
ton of refrigeration	—	TR

Power is energy per unit of time: 1 W = 1 J/s. 1 metric horsepower = 75 kgf·m/s = 735.5 W (lifting 75 kg by 1 m in 1 s); 1 hp = 745.7 W.

Energy = power × time: a 60 W bulb left on for 5 h uses 300 Wh = 0.3 kWh.

A person can sustain about 75-100 W of mechanical work for hours; a draught horse about 500-750 W on a sustained basis.
The ton of refrigeration (12,000 BTU/h, about 3.5 kW) is the power needed to melt a US short ton of ice in 24 h and is used for air conditioning.