Power Converter

Created by Hanna Pamuła, PhD candidate
Reviewed by Bogna Szyk
Last updated: Apr 06, 2022

If you are wondering how to convert horsepower to watts or watts to calories, this power converter is a tool for you. Change quickly between twenty different power units by typing one value into the calculator. If you are curious about power conversion factors, BTU to watts conversion or you just want to know what a watt actually is, keep scrolling and you'll find the answer.

You can also check out our dbm to watts calculator to learn how to convert between decibels per milliwatt (dBm) and watts.

Power units

In this power converter, we implemented twenty popular force units. The most common ones are the SI unit watt and its nine most frequently used multiples:

  • picowatts (pW)
  • nanowatts (nW)
  • microwatts (µW)
  • milliwatts (mW)
  • kilowatts (kW)
  • megawatts (MW)
  • gigawatts (GW)
  • terawatts (TW)
  • petawatts (PW)

And ten other power units, including 5 types of horsepowers:

  • British thermal units per hour (Btu/h, sometimes known as Btu or BTU)
  • mechanical horsepowers (hp(l))
  • metric horsepowers (hp(M))
  • electric horsepowers (hp(E))
  • air horsepowers (ahp)
  • boiler horsepowers (hp(S))
  • kilocalories per hour (kcal/h)
  • kilocalories per minute (kcal/min)
  • ergs per second (erg/s)
  • ton of refrigeration (TR)

Power conversions

The formulas implemented in our power converter to change between any from 11 popular power units into watts look as follows:

  • 1 picowatt [pW] = 0.000000000001 watt [W] = 10⁻¹² watt [W]

  • 1 nanowatt [nW] = 0.000000001 watt [W] = 10⁻⁹ watt [W]

  • 1 microwatt [µW] = 0.000001 watt [W] = 10⁻⁶ watt [W]

  • 1 milliwatt [mW] = 0.001 watt [W] = 10⁻³ watt [W]

  • 1 kilowatt [kW] = 1000 watt [W] = 10³ watt [W]

  • 1 megawatt [MW] = 1000000 watt [W] = 10⁶ watt [W]

  • 1 gigawatt [GW] = 1000000000 watt [W] = 10⁹ watt [W]

  • 1 terawatt [TW] = 1000000000000 watt [W] = 10¹² watt [W]

  • 1 petawatt [PW] = 1000000000000000 watt [W] = 10¹⁵ watt [W]

  • 1 Btu/hour [Btu/h] = 0.293071 watt [W]

  • 1 mechanical horsepower [hp(l)] = 745.6998715823 watt [W]

  • 1 metric horsepower [hp(M)] = 735.49875 watt [W]

  • 1 electric horsepower [hp(E)] = 746 watt [W]

  • 1 air horsepower [ahp] = 745.69987158 watt [W]

  • 1 boiler horsepower [hp(S)] = 9812.5 watt [W]

  • 1 kilocalorie/hour [kcal/h] = 1.162222 watt [W]

  • 1 kilocalorie/minute [kcal/min] = 69.733333 watt [W]

  • 1 erg/second [erg/s] = 10⁻⁷ watt [W]

  • 1 ton of refrigeration [TR] = 3516.8528420667 watt [W]

What is a watt?

Watt is a basic SI unit of power and it is equal to one joule per second.
The basic definition of a watt unit looks as follows:

If object's velocity is held constant at one meter per second against a constant opposing force of one newton, the rate at which work is done is 1 watt.

1 watt = 1 joules / second

1 W = 1 * J / s = 1 * N * m / s = 1 * kg * m² / s³ in basic SI units

Watt can also be defined in terms of electromagnetism - electric power is the rate at which electrical work is performed when a current of one ampere flows across an electrical potential difference of one volt.

1 watt = 1 ampere * 1 volt

To calculate the relationship between the real, reactive, and apparent power in an AC circuit, check out this power factor calculator.

SI multiples for watt (W)

What's the relationship between a watt and e.g. megawatt? Or how much power is 1 Gigawatt? The table below presents the basic SI multiples for watt and their applications.


SI symbol



10⁻²⁴ W



10⁻²¹ W



10⁻¹⁸ W



sound intensity in water

10⁻¹⁵ W



radio and radar receivers

10⁻¹² W



radio and radar receivers, acoustics, radio astronomy

10⁻⁹ W



radio and radar receivers

10⁻⁶ W



radio and radar receivers, medical instrumentation systems (EEG,ECG)

10⁻³ W



acoustics (dBm), hearing aids, laser pointers

10⁻2 W



10⁻¹ W



1 W



bulbs 💡, different electronic devices

10¹ W



10² W



10³ W



different electronic devices, engines, motors, tools, machines, heaters, broadcast radio, television transmitters

10⁶ W



large electric motors (e.g. cruisers, submarines), server clusters, supercolliders, power plants

10⁹ W



large power plants or power grids

10¹² W



total power used by humans worldwide, average lightning strike peaks at 1 TW

10¹⁵ W



total power of sunlight striking Earth's atmosphere, current generation of lasers (but for order of picoseconds)

10¹⁸ W



10²¹ W



10²⁴ W



Horsepower 🐎 and its types

Horsepower unit explanation

The unit was introduced in the 18th century to compare steam engines with the power of draft horses. Later it was expanded to different types, as new engines and machines were appearing. The most often used units are mechanical and metric horsepowers, equal to ~ 745.7 watts and ~ 735.5 watts respectively.

  1. (Imperial) mechanical horsepower

A mechanical horsepower was defined by James Watt to be equal to precisely 33,000 ft lbf/min. Watt found this value by an estimation of horse performance - he observed how many times a horse could turn a mill wheel with a given diameter, then he assumed the force with which the horse pull, and rounded the obtained value to get the final 33,000 ft lbf/min.

Thus, horse work rate is about one hp in sustained activity (but it's rather a maximum value, average horse can sustain 2/3 hp for a working day1,2 ), in contrast to a possible peak power of almost 15 hp. For a standard healthy human, these values are equal to about 1.2 hp (0.89 kW) of peak and 0.1 hp of sustained work. Athletes such as e.g. Usain Bolt can produce even 3.5 hp (2.6 kW) for a short time.

  1. Metric horsepower

One metric horsepower is defined as the power needed to raise a mass of 75 kilograms against the Earth's gravitational force over a distance of one meter in one second.

  1. Electric horsepower

Slightly higher value than mechanical horsepower, electrical equivalent equal to exactly 746 watts.

  1. Air horsepower

It's the horsepower required to move a given volume of air against given pressure, assuming 100% efficiency. It may be used in air conditioning, heating, and ventilating systems.

ahp = flow rate (cubic feet / minute) * pressure (inches water column) / 6,356

Air horsepower is equal in value to mechanical horsepower and hydraulic horsepower.

  1. Boiler horsepower

Boiler horsepower (hp(S), also abbreviated as bhp or BHP) is usually defined as the amount of energy required to produce 34.5 pounds (15.65 kg) of steam per hour at pressure 0 psig (0 bar) and temperature 212°F (100°C). It was used in the USA, Australia, and New Zealand, but the formula differed among these countries. In our power conversion tool, we assumed that 1 boiler horsepower = 9,812.5 W. However, values such as 9810.55407 W, 9809.5 W or 9809 W may be found in some sources.

Power converter - how to use

Assume you want to convert BTU to watts to find the power of your heating or cooling system in watts.

  1. Select the unit you want to convert. In our case, it's Btu/h (sometimes abbreviated as just Btu or BTU).
  2. Enter the value into the right box. If it's not visible, click on the power unit name in proper section to select the unit from a drop-down list. Let's say that we have a 10,000 BTU air conditioner.
  3. Here it is! The power converter displays our value in different units, including watts. So we found out that 1,0000 Btu/h is equal to 2,931 watts.

Now it's your turn, try this power converter yourself! For example, compare car engine performance 🚗 with another device (why not the vacuum cleaner?) by changing the car's horsepowers to watts.

Hanna Pamuła, PhD candidate
SI units
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