Power Converter

About this Power Converter

The Power Converter changes power ratings between common units used for electrical equipment, engines, heating and mechanical specifications. Power measures the rate at which energy is transferred or used.

Power measures the rate of doing work or transferring energy, and it appears in very different everyday contexts, from a lightbulb's wattage, to a car engine's horsepower rating, to an air conditioner's BTU-per-hour cooling capacity.

For related tasks, try Energy Converter, Pressure Converter or Force Converter.

How the calculation works

Conversions use watts as the internal base unit with fixed reference factors.

How to use it

1. Enter a power value.
2. Choose source and target units.
3. Convert to see the equivalent rating.

Practical example

1 kilowatt equals 1,000 watts and is approximately 1.341 mechanical horsepower.

1 horsepower equals approximately 745.7 watts, so a 200-horsepower car engine produces roughly 149,140 watts, or about 149.1 kilowatts, of power at that rating.

Things to double-check

Power (watts, horsepower) and energy (watt-hours, joules) are related but different concepts; power is the rate at which energy is used, so a 1,000-watt appliance run for 2 hours consumes 2,000 watt-hours of energy.

Watts, kilowatts and horsepower

One kilowatt is 1,000 watts, which is about 1.341 mechanical horsepower. One mechanical horsepower is 745.7 watts, and one metric horsepower (PS) is 735.5 watts. One kilowatt is also about 3,412 BTU per hour. A 100 horsepower engine produces about 74.6 kW. The kind of horsepower matters for precise work: the difference between the two is about 1.4 percent.

Heating and cooling ratings

Air conditioners are rated in BTU per hour and in tons of refrigeration, where one ton is 12,000 BTU per hour, or about 3.517 kW. A 1.5 ton unit therefore has about 5.3 kW of cooling capacity. When you size a unit, consider room size, insulation, sun exposure and climate, not only the number. An oversized unit cycles on and off and cools less evenly.

Appliances and bills

Power in watts is how fast energy is used, and energy in kilowatt-hours is power times time. A 2 kW kettle takes about 3 minutes to boil, using 0.1 kWh. A 100 W bulb left on for 10 hours uses 1 kWh. Converting ratings helps you compare devices and estimate running costs, and checking the power rating on a plug or generator stops you from overloading it.

Engines, motors and generators

Engine power is quoted in horsepower or kilowatts depending on the country, and electric motors are usually in kilowatts. A generator's rating is also in kVA or kW, and these differ by the power factor. When you pick a generator for appliances, add up the running wattage and allow extra for motor start-up surges, which can be two to three times the running power. Convert all figures to kilowatts before you add them.

Sizing a generator or inverter

To choose a backup power unit, add up the running power of what you want to run and allow for start-up surges. For example, a fridge at 150 W, a fan at 75 W and lights at 60 W total 285 W. The fridge motor may briefly draw about three times its running power when it starts, so plan for around 450 W of surge. A unit with a rating of at least 600 to 1,000 W would cover this with a margin. Convert all ratings to watts or kilowatts first, and check whether a rating is in watts or volt-amperes.

Comparing motors and engines

Power ratings let you compare very different machines. A 75 kilowatt electric motor produces about 100.6 horsepower, and a 150 kilowatt motor in a car is about 201 horsepower. A small kitchen blender may use 500 watts, a hairdryer about 1,800 watts and an electric kettle about 2,000 watts. When you compare vehicles, remember that power is not the same as torque, which describes the turning force, and that a rating can be given at different engine speeds. Convert to one unit before you compare specifications from different makers.

Worked examples

An engine of 85 horsepower produces about 63.4 kilowatts, since one horsepower is 0.7457 kilowatts. A 2,400 watt heater is about 3.2 horsepower. An air conditioner rated at 18,000 BTU per hour removes heat at about 5.28 kilowatts. For any conversion, convert to watts first and then to the target unit. Mechanical and metric horsepower differ slightly, so for precise work note which one is used. Writing the unit after every number prevents mistakes, such as treating a kilowatt rating as watts.

Power versus energy on your bill

Power is a rate, and energy is power multiplied by time. A 1.2 kilowatt microwave used for 5 minutes consumes 0.1 kilowatt-hours. A 100 watt bulb running for 8 hours uses 0.8 kilowatt-hours, while a 10 watt LED giving similar light uses 0.08, saving 0.72 each night, or about 263 kilowatt-hours a year if used daily. Bills charge for energy, not power, although some commercial tariffs also include a charge based on peak demand. Knowing the power of each appliance lets you estimate costs and find the devices that matter.

Efficiency and losses

No machine converts all input power into useful output. An electric motor that is 90 percent efficient delivers 0.9 kilowatts of mechanical power for 1.0 kilowatt of electricity, with the rest lost as heat. Petrol engines convert only around a quarter to a third of the fuel energy into motion. When you compare a rated output power with the power drawn from the supply, remember the efficiency. For alternating current equipment, the power factor also matters, since apparent power in volt-amperes can be larger than real power in watts.

Circuits, breakers and limits

Every circuit has a maximum power set by its voltage and current rating. A 10 amp circuit on 230 volts supports about 2.3 kilowatts, and a 15 amp circuit about 3.45 kilowatts. In the United States, a 15 amp circuit at 120 volts supports 1,800 watts, and continuous loads are usually kept to 80 percent of that. Running a heater, kettle and iron on one circuit can exceed the limit and trip the breaker. Always check the rating of cables, sockets and extension leads, and ask a qualified electrician for new circuits.

Key terms explained

A watt is one joule of energy per second, and a kilowatt is a thousand watts. Horsepower comes in a mechanical version of 745.7 watts and a metric version of 735.5 watts. A British thermal unit per hour measures heating and cooling power, and a ton of refrigeration is 12,000 of them per hour. Peak power is the highest output a machine can reach, and continuous power is the output it can sustain. Efficiency is the ratio of useful output to input.

Rules of thumb and quick checks

One kilowatt is about 1.34 horsepower, and one horsepower is about 0.746 kilowatts. One kilowatt is about 3,412 BTU per hour. A 2 kilowatt heater running for one hour uses 2 kilowatt-hours. To size an electrical circuit, divide power in watts by voltage to find the current in amperes. Add a safety margin of 20 to 25 percent, and remember that motors need extra current when they start.

Quick reference and sanity checks

Useful anchors are one kilowatt equals 1,000 watts and one megawatt equals 1,000 kilowatts. A mechanical horsepower is about 745.7 watts, while a metric horsepower is about 735.5 watts, so check which one a specification uses. Power is the rate of using energy, so a 2 kilowatt heater that runs for 3 hours uses 6 kilowatt hours of energy, and you should not confuse the two units. Nameplates on equipment show the power drawn under stated conditions, and real use can differ. If you size a generator or a solar system, add a safety margin and ask a qualified electrician to confirm the design.

Frequently asked questions