Power is energy per unit time — the rate of doing work. The SI unit is the watt (W = J/s). James Watt coined "horsepower" in the 18th century to market his steam engines against horse-drawn alternatives: he defined 1 hp as lifting 550 foot-pounds per second, equivalent to 745.7 watts. Today, car engines are rated in kilowatts (metric markets) or horsepower (US). 100 kW ≈ 134 hp is a useful reference point.
Household appliances are rated in watts: an electric kettle draws 2,000–3,000 W; a modern LED bulb ≈ 10 W (replacing a 60 W incandescent). Solar panels are rated in watts peak (Wp). Data centres and power grids operate at megawatt to gigawatt scales. The human body at rest produces about 80 W of metabolic heat; an elite cyclist can sustain 400–500 W at peak effort for brief periods.
| 1 hp | = 745.7 W |
| 1 kW | = 1.341 hp |
| 1 BTU/hr | = 0.2931 W |
| 100 W × 1 hr | = 0.1 kWh |
| 1 MW | = 1,000 kW |
| 1 MW | = 1,341 hp |