Power Converter

Car engines are rated in horsepower or kilowatts, air conditioners in BTU per hour, and appliances in watts. This converter lines them up: watts, kilowatts, megawatts, mechanical and metric horsepower, BTU per hour and calories per second. Pick the units and the whole table updates.

How to use it

1. Enter the power value.
2. Choose the unit you have and the one you want.
3. Read the headline answer and the other units.
4. Swap units to reverse it.

When it helps

• Comparing an engine quoted in horsepower with one in kilowatts.
• Checking the cooling capacity of an air conditioner.
• Sizing a generator against appliance ratings.
• Electrical and mechanical engineering work.

Good to know

• Mechanical horsepower is about 745.7 watts, while metric horsepower (PS) is about 735.5 watts, so figures differ slightly by market.
• A 12,000 BTU/h air conditioner (called "one ton") is about 3.5 kW of cooling.
• Power is energy per unit time. If you want energy itself, use the energy converter.

How the conversion works

Power is how fast energy is used or produced, and the base unit is the watt, one joule per second. A kilowatt is 1,000 watts, a megawatt a million. Mechanical horsepower is defined as 745.7 watts, a figure James Watt chose after measuring how fast a dray horse could lift coal. Metric horsepower, or PS, is 735.5 watts.

BTU per hour measures heat output and is common on air conditioners and furnaces. One BTU per hour is about 0.293 watts, so a 12,000 BTU/h unit moves about 3.5 kilowatts of heat.

Worked example: an engine and a generator

A car makes 150 horsepower. Enter 150 with hp selected: that's about 111.9 kW, or 152 PS. A generator rated 5 kW can therefore run roughly the output of a small engine, in electrical terms, though motors and engines convert energy at different efficiencies.

For appliances, a 1,500-watt kettle is 1.5 kW, and running it for 10 minutes uses 0.25 kWh. A 100 W bulb for ten hours uses 1 kWh, which is the standard unit your electricity company bills in.

Power versus energy

People often mix the two. A kilowatt is a rate, and a kilowatt-hour is an amount: using 1 kW for one hour. Your bill is for energy. The rating on an appliance is its power. Knowing which one a figure refers to helps you avoid nonsense claims such as a battery that "produces 5 kW".

If you need to convert energy, use the Energy Converter. This tool is for rates only.

Typical values

A laptop uses 30 to 60 watts, a fridge about 100, a microwave 1,000 to 1,500, an electric shower 7 to 10 kilowatts, and a family car engine 70 to 150 kilowatts. A large wind turbine produces 2 to 5 megawatts, and a big power station 1,000 megawatts or more.

These ranges give you a feel for scale when checking a conversion. If your answer puts a phone charger at 500 horsepower, a unit is wrong.

Questions