Power Converter

This converter changes a power figure from one unit into another — kilowatts and megawatts, the four different horsepowers still in circulation, BTU per hour, calories per second, foot-pounds force per minute, ergs per second and milliwatts. Power is the rate at which energy is used or produced, so every unit here is energy divided by time, however differently they are dressed up.

It is worth being clear about what this tool is not. It converts units of power. It is not about power supplies, adapters or voltage conversion for travel; if you landed here looking for that, this page will not help.

Which horsepower did they mean?

"Horsepower" is not one number. There are four in common use and they disagree by enough to matter:

• Mechanical horsepower (550 ft·lbf/s) = 745.700 W — the standard in the UK and US
• Metric horsepower (PS, ch, cv) = 735.499 W — used across continental Europe and in most car brochures
• Electric horsepower = 746 W exactly — used for rating motors
• Boiler horsepower = 9,809.5 W — a completely different quantity, used for steam plant

The gap between mechanical and metric horsepower is about 1.4%. On a 300 hp car that is four horsepower, which is why the same engine is sometimes quoted with two different figures in two different markets. Boiler horsepower is thirteen times larger than the others and catches people out badly.

A worked example: engine output

A manufacturer quotes an engine at 150 mechanical horsepower and you want kilowatts for a European registration document.

• 150 × 745.6999 W = 111,855 W
• = 111.85 kW

Running it back the other way, 2.5 kW of continuous output is 2,500 ÷ 745.6999 = 3.35 hp — about right for a decent workshop compressor.

A second example: air conditioning

Air conditioning and heating are usually sold in BTU per hour in North America and in kilowatts elsewhere. A 12,000 BTU/h unit — what the trade calls "one ton" of cooling — is:

• 12,000 × 0.29307 W per BTU/h = 3,516.9 W
• = 3.517 kW

So a 3.5 kW split system and a 12,000 BTU unit are the same machine described two ways.

How to use it

1. Enter the number you have.
2. Pick its unit from the first list — if it is horsepower, check which horsepower.
3. Pick the unit you want from the second list.
4. The result updates as you type.

Power is not energy

The most common error with these units has nothing to do with the arithmetic. Power is a rate; energy is an amount. A 2 kW heater running for three hours uses 6 kilowatt-hours of energy. The kilowatt is power, the kilowatt-hour is energy, and they are not interchangeable — you cannot convert between them without knowing the duration.

Your electricity bill is in kilowatt-hours because you are charged for energy consumed. An appliance label is in watts because it tells you the rate while it is running. If a conversion looks wrong by a factor that happens to equal a number of hours, this is usually why.

Reading an appliance or solar specification

Domestic equipment is labelled in watts, and the numbers span a wide range: an LED bulb is around 9 W, a laptop charger 65 W, a kettle 3,000 W, an electric shower 9,500 W. Solar panels are rated in watts-peak, the output under standard test conditions rather than on a British afternoon in November.

Where the converter earns its place is in comparing across systems — a generator advertised at 6.5 hp against an inverter rated in kilowatts, or a wood burner quoted in BTU per hour against a heat pump quoted in kilowatts. Putting both onto the same unit is usually enough to show that two products which looked different are closely matched, or that one is nowhere near the duty you need.

Frequently asked questions

How many watts are in one horsepower?

745.7 watts for mechanical horsepower, 735.5 for metric. For rough mental arithmetic, three quarters of a kilowatt is close enough for either.

Why do European and American car specs differ for the same engine?

Usually because one is quoted in metric horsepower (PS) and the other in mechanical horsepower, a 1.4% difference. Sometimes it is also a different test standard, which the units cannot account for.

What is a BTU per hour?

A British thermal unit is the energy needed to raise a pound of water by one degree Fahrenheit. Per hour makes it a rate, which is why heating and cooling equipment is rated in it. One BTU/h is about 0.293 watts.

Is kVA the same as kW?

No, and this converter does not handle kVA. Kilovolt-amperes measure apparent power in an AC circuit; kilowatts measure real power. Converting between them needs the power factor of the specific load, which is not a fixed constant.

Why is there no plain "watt" in the list?

The list covers milliwatts, kilowatts and megawatts. For a value in watts, enter it as kilowatts divided by a thousand, or use milliwatts and multiply — the underlying factors are the same either way.

Related tools

• Torque Converter — engine torque figures usually arrive alongside power figures.
• Pressure Converter — for the other half of most mechanical specifications.
• Time Unit Converter — useful when turning a rate into a total.
• Temperature Converter — heating and cooling calculations need both.