About Power
Power is the rate at which energy is transferred or converted. The watt (W) is the SI unit, equal to one joule per second.
Horsepower (hp) originated from comparing steam engine output to horse work — 1 mechanical hp = 745.7 W.
Pro Tips
Power (W) = Voltage (V) × Current (A) — useful when sizing electrical circuits.
Air conditioner capacity is often rated in BTU/hr — 12,000 BTU/hr equals 1 "ton" of cooling.