Introduction / overview

Power Converter turns one power value into equivalent values across the units engineers, technicians, and hobbyists actually reach for — watts and its SI multiples, mechanical and metric horsepower, BTU per hour, kilocalories per hour, ergs per second, and tons of refrigeration.

Enter a value in any field, and the equivalent power is recalculated across every unit at once. You can start from the unit you have and read the unit you need, in either direction.

It is the kind of tool you reach for when a motor datasheet lists power in horsepower but your spec sheet expects kilowatts, when an appliance label gives watts and you want the cooling load in BTU per hour, or when a physics problem states a value in ergs per second and you need watts. The three main fields cover the SI family, and the remaining fields cover horsepower, heat-rate, and refrigeration units.

How to use / quick start

1Find the field for the unit you already know, and enter the power value there.
2Read the equivalent value in the unit you need. Every other field represents the same power, so nothing else has to be typed.
3If the unit you need is not the field's default, switch that field's unit selector — for example, the watts field can display picowatts through petawatts, and the horsepower field offers mechanical, metric, electric, and air horsepower.

Example: converting a 1 mechanical-horsepower motor to watts and BTU/h

Type $1$ in the "Power in mechanical horsepowers" field. The calculator multiplies by the exact mechanical-horsepower factor declared for this tool:

1\ \mathrm{hp(l)}

\times

745.69987158227022

\ \mathrm{W/hp(l)}

=

745.7\ \mathrm{W}

The same power is shown at the same time as $0.7457\ \mathrm{kW}$ , $2{,}544.43\ \mathrm{BTU/h}$ , and $1.01387\ \mathrm{hp(M)}$ , so you can read every unit you need in one pass.

Calculation method

Internally, every field is converted through watts. If $P_s$ is the value you entered and $f_s$ is the number of watts in one source unit, the shared power is:

P_{\mathrm{W}} = P_s \times f_s

P_t

=

P_{\mathrm{W}} \div f_t

=

P_s \times f_s \div f_t

How to read the method:

$P_{\mathrm{W}}$ is the shared power expressed in watts.
$f_s$ and $f_t$ are the declared watts-per-unit factors for the source and target units, such as $745.69987158227022\ \mathrm{W/hp(l)}$ or $0.293071\ \mathrm{W/(BTU/h)}$ .
You can enter any editable field, so the same relationship works in both directions rather than only from watts outward.

Unit	Watts per unit
1 mechanical horsepower (hp(l))	745.69987158227022 W
1 metric horsepower (hp(M))	735.49875 W
1 electric horsepower (hp(E))	746 W
1 BTU per hour (BTU/h)	0.2930710701722222… W
1 kcal per hour (kcal/h)	1.1622222222222222… W
1 kcal per minute (kcal/min)	69.73333333333333… W
1 erg per second (erg/s)	0.0000001 W
1 ton of refrigeration (TR)	3,516.852842066666… W

The SI multiples of the watt are exact powers of ten, and the horsepower, erg, and heat-rate factors above are the exact definitions this tool uses. The BTU/h, kcal/h, kcal/min, and ton-of-refrigeration values repeat because they are exact rational ratios written out in decimal.

Power, energy, and horsepower

Power is not energy

Power is the rate at which energy is transferred. The watt is a derived SI unit equal to one joule per second:

1\ \mathrm{W}

=

1\ \mathrm{J/s}

Energy is power multiplied by time. A 1,000 W appliance running for one hour uses $1\ \mathrm{kWh}$ of energy, which is why electricity bills charge in kilowatt-hours rather than watts. This converter works in power only — if you need to turn a power rating into energy over time, multiply the watts by the hours yourself.

Why there are three common horsepower units

Horsepower predates the watt, and different industries standardized on different definitions. Mechanical horsepower is defined as $33{,}000\ \mathrm{ft{\cdot}lbf/min}$ , which works out to exactly $745.69987158227022\ \mathrm{W}$ . Metric horsepower is defined as $75\ \mathrm{kgf{\cdot}m/s}$ , exactly $735.49875\ \mathrm{W}$ . Electric horsepower is exactly $746\ \mathrm{W}$ .

Mechanical horsepower (hp(l)) is the classic engine rating used in the US and UK for motors, pumps, and vehicles.
Metric horsepower (hp(M)) is the DIN rating used across Europe; it is about $1.4\%$ smaller than mechanical horsepower.
Electric horsepower (hp(E)) appears on electric-motor nameplates.

If you have torque and rotational speed instead of a power rating, the Torque to hp Calculator computes power from those two values directly.

Heat-rate units in heating and cooling

BTU per hour is the standard heating and cooling rate in North America, and tons of refrigeration is the classic cooling-capacity unit. One ton of refrigeration equals $12{,}000\ \mathrm{BTU/h}$ , about $3{,}517\ \mathrm{W}$ . A window air conditioner rated at 12,000 BTU/h is therefore drawing roughly 3.5 kW of cooling power, which is why the calculator's BTU/h and tons-of-refrigeration fields are useful when comparing HVAC equipment against electrical ratings.

Frequently asked questions

Q

How many watts is one horsepower?

It depends on which horsepower definition you mean:

Mechanical horsepower = $745.7\ \mathrm{W}$
Metric horsepower = $735.5\ \mathrm{W}$
Electric horsepower = $746\ \mathrm{W}$

The calculator's horsepower field defaults to mechanical horsepower, and you can switch it to the metric or electric definition with the unit selector.

Q

Why does the calculator show a negative power?

Power can flow in either direction, so negative values are physically meaningful. Regenerative braking, reverse power flow in a generator, and power absorbed by a load are all commonly written as negative. This converter keeps the sign visible instead of treating it as an error, so a value like $-1.5\ \mathrm{kW}$ converts normally to $-2.01153\ \mathrm{hp(l)}$ .

Power Converter

Introduction / overview

How to use / quick start

Calculation method

Frequently asked questions

How many watts is one horsepower?

Why does the calculator show a negative power?

Limitations

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