A power converter changes a rate of energy use from one unit into another — kilowatts to horsepower, or BTU per hour to watts. Power is energy per unit of time, so it describes how fast work is done rather than how much total energy is involved. This tool covers watts, kilowatts, megawatts, both kinds of horsepower, BTU per hour and foot-pounds per second.
The formula. Every value passes through one base unit, the watt (one joule per second). The factors are 1 kW = 1,000 W, 1 MW = 1,000,000 W, 1 mechanical horsepower = 745.6999 W, 1 metric horsepower (PS) = 735.49875 W, 1 BTU/h = 0.293071 W and 1 ft·lb/s = 1.355818 W. Note the two horsepower definitions differ by about 1.4%, which is why the same car is advertised as 300 hp in the US and 304 PS in Germany.
Worked example. A 150 hp engine is 150 × 745.6999 = 111,855 W, or 111.85 kW. A 1.5 kW kettle is 1,500 ÷ 745.6999 ≈ 2.01 hp. And a 12,000 BTU/h air-conditioner is 12,000 × 0.293071 = 3,517 W, or about 3.5 kW — the number you need when checking whether a circuit can carry it.
Where it's used. Vehicle output is quoted in hp, PS or kW depending on the market; appliances and solar panels are rated in watts and kilowatts; heating and cooling equipment uses BTU per hour or kW; and power stations are measured in megawatts. Keep power and energy distinct: a kilowatt is a rate, a kilowatt-hour is an amount — a 2 kW heater run for 30 minutes consumes 1 kWh.