Power Converter

What this calculates

Enter a power in watts, kilowatts, megawatts, mechanical horsepower, metric horsepower or BTU per hour, and this shows it in all six at once, with each conversion worked through. It also places the power on a scale running from 1 W to 1 PW, in bands a thousand times apart, each shown in mechanical horsepower as well, so the size of a power can be read at a glance.

It converts a power as it is. It does not convert power into energy, such as kilowatts into kilowatt-hours.

The formula

Every conversion goes through watts: into watts with one unit's size, then out again with another's.

Each unit's size comes from its definition.

A mechanical horsepower is 550 foot-pounds-force per second, and a metric horsepower is 75 kilograms-force metres per second. The mechanical horsepower has seventeen significant figures, so it is carried as 745.6998715822702 W, and a BTU per hour does not come out as a terminating decimal in watts. Where a watt figure runs past ten significant figures, the working shows it to ten, marked ≈, while every result is worked out from the exact value.

Worked example

Some conversions come out exact. 1 PS is exactly 735.49875 W, and 1 MW is exactly 1,000 kW.

When to use it, and the mistakes to avoid

Use it when a power is given in a unit you do not think in, when comparing a figure in horsepower with one in kilowatts, and when a power is given in BTU per hour.

The mistakes that cost the most:

• Mixing up kW and kWh. A kilowatt is power, a rate; a kilowatt-hour is energy, an amount. A steady 1 kW for one hour is 1 kWh, but a kilowatt figure on its own cannot be converted into kilowatt-hours without knowing for how long. This converter takes powers only.
• Treating hp and PS as the same. They are close but not equal: 1 PS is 0.9863201 hp, and 1 hp is 1.01387 PS. At 100 PS the difference is already visible — 98.63201 hp. Check which horsepower a figure is in.
• Using × 0.75 as if it were exact. A mechanical horsepower is 0.7456999 kW, not 0.75. The rough rule turns 100 hp into 75 kW against the true 74.56999 kW. Close for a rough sense, but not an answer.
• Mixing up W and kW. A kilowatt is 1,000 W, so 1,500 W is 1.5 kW, not 1,500 kW. Check which of the two a figure is in before comparing it with another.
• Expecting every digit of every result. Results are shown to seven significant figures — 1 kW shows as 1.341022 hp — while each is worked out from the exact value. A power far too small for a unit, such as 0.000000000000001 BTU/h in watts, kilowatts, megawatts or either horsepower, reads as 0, and a note under the result says so rather than leaving a silent zero.
• Entering a negative power. This version converts a power as it is and does not treat a direction of flow, where a figure below zero would make sense, so it refuses a negative value with a message instead of converting it.

FAQ