Power is energy per unit of time
One watt equals one joule per second. A kilowatt is 1,000 watts and a megawatt is one million watts. Power describes a rate; energy describes the accumulated amount transferred over time.
Horsepower conventions
This tool uses mechanical horsepower, approximately 745.6998715822702 watts. Metric horsepower and electrical horsepower use different definitions, so confirm the convention in a vehicle, motor, or equipment specification.
Heating and cooling ratings
BTU per hour is a power unit even though the “per hour” is sometimes omitted in casual descriptions. This tool uses the International Table BTU, making 1 BTU/h approximately 0.2930710701722222 watt.
Read Energy and Power Units Explained for examples involving appliance ratings and kilowatt-hours.
Convert a motor rating without changing its meaning
A mechanical output rating of 5 hp converts to about 3,728.5 W, or 3.7285 kW, using the horsepower definition selected here. That is not automatically the electrical input required by the motor. If the motor and drive are less than 100% efficient, input power is higher than mechanical output; voltage, current, power factor, load, and phase also matter. Record whether the source value is input, shaft output, heat output, nominal capacity, or measured demand before comparing equipment.
Turn power into energy only with time
A constant 1.5 kW appliance running for two hours uses 3 kWh. If a thermostat lets it run at full power for only half the interval, a simple duty-cycle estimate is 1.5 kW × 2 h × 0.5 = 1.5 kWh. The converter does not perform that time integration. The watt, joule, and derived SI relationships are listed in the NIST Guide to SI derived units.
A unit conversion does not account for input versus output power, duty cycle, efficiency, or operating conditions. Use the definitions required by the relevant standard.