How To Convert Power Units

Power is the rate at which work is done or energy is transferred. The SI unit is the watt, which is one joule per second, and it is the base for every other power unit.

A kilowatt is a thousand watts and a megawatt is a million watts. These SI-prefixed units appear on appliance labels, engine specs and power station ratings.

Horsepower is a non-SI unit that comes in two main versions. Mechanical horsepower, used for US car and motor ratings, equals 550 foot-pounds per second, or about 745.6998716 watts.

Metric horsepower, written PS in German and CV in French, equals 75 kilogram-force metres per second, or about 735.49875 watts. The two differ by about 1.4 percent, which is enough to matter when comparing specifications.

A BTU per hour measures a rate of heat transfer and equals about 0.29307107 watts. It is standard in heating, ventilation and air conditioning.

A foot-pound per second is the rate of doing one foot-pound of work each second and equals about 1.3558179483 watts. It links directly to the definition of mechanical horsepower.

The converter expresses every unit as a size in watts and converts by multiplying the value by the source factor and dividing by the target factor. That keeps every pair consistent.

The most common automotive conversion is kilowatts to mechanical horsepower: multiply kilowatts by 1000 and divide by 745.6998716, which gives about 1.3410221 horsepower per kilowatt.

Going the other way, multiply horsepower by 745.6998716 and divide by 1000 to get kilowatts, which is about 0.7457 kilowatts per horsepower.

For heating and cooling, the conversion between watts and BTU per hour uses the factor 0.29307107. A 12,000 BTU per hour air conditioner is about 3,517 watts, or 3.5 kilowatts.

A frequent error is to use the wrong horsepower. A European car rated at 150 PS produces about 148 mechanical horsepower, a small but real difference that matters in comparisons.

Another trap is rounding early. One kilowatt is 1.3410221 horsepower, not 1.34, and the difference accumulates when several conversions are chained.

Appliance labels mix watts and kilowatts, and a heater rated at 2 kilowatts draws 2000 watts. Knowing the wattage lets you estimate running cost from the hours used and the price per kilowatt-hour.

Engine specifications are a minefield of horsepower and kilowatts. A 150 kilowatt motor is about 201 mechanical horsepower, and quoting the wrong unit can make two similar machines look very different.

Air conditioning is sized in BTU per hour in the United States and in kilowatts almost everywhere else. A 12,000 BTU per hour unit is about 3.5 kilowatts, the figure on a European specification sheet.

Mechanical power is torque multiplied by angular speed, so a torque conversion and a speed conversion sit behind every power figure. That is why power, torque and frequency converters are usually used together.

Because power is a rate, it scales with time in a way energy does not. Running a 1 kilowatt device for two hours uses 2 kilowatt-hours, while a 2 kilowatt device for one hour uses the same energy.

Solar installations are quoted in kilowatts of peak capacity and in kilowatt-hours of production, so both the power and the energy units appear on one specification sheet. Converting between them needs the hours of sunshine.

Motor nameplates often give both the kilowatt rating and the horsepower, precisely because the two markets use different units. Checking the pair is a quick way to confirm a conversion is correct.

The calculator models the fixed relationships between the units and nothing more. It does not know whether a rating is mechanical or metric horsepower, so choose the right option and treat the output as exact arithmetic.