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.

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.

Value SI symbol Name Applications
10−24 W yW yoctowatt
10−21 W zW zeptowatt
10−18 W aW attowatt sound intensity in water
10−15 W fW femtowatt radio and radar receivers
10−12 W pW picowatt radio and radar receivers, acoustics, radio astronomy
10−9 W nW nanowatt radio and radar receivers
10−6 W µW microwatt radio and radar receivers
medical instrumentation systems (EEG,ECG)
10−3 W mW milliwatt acoustics (dBm), hearing aids, laser pointers
10−2 W cW centiwatt
10−1 W dW deciwatt
1 W W watt bulbs 💡, different electronic devices
101 W daW decawatt
102 W hW hectowatt
103 W kW kilowatt different electronic devices, engines, motors, tools, machines,
heaters, broadcast radio, television transmitters
106 W MW megawatt large electric motors (e.g. cruisers, submarines),
server clusters, supercolliders, power plants
109 W GW gigawatt large power plants or power grids
1012 W TW terawatt total power used by humans worldwide,
average lightning strike peaks at 1 TW
1015 W PW petawatt total power of sunlight striking Earth's atmosphere,
current generation of lasers (but for order of picoseconds)
1018 W EW exawatt
1021 W ZW zettawatt
1024 W YW yottawatt

Horsepower 🐎 and its types

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.

Hanna Pamuła, PhD student

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Power Units. Converter | Definitions