Everyday Life · Unit Conversions · Energy & Power
Power Unit Converter
Converts a power value from one unit to all major power units including watts, kilowatts, megawatts, horsepower, BTU/hr, and foot-pounds per second.
Calculator
Formula
P_out is the converted power value in the target unit. P_in is the original power value. F_in is the conversion factor of the input unit to watts (i.e., how many watts equal one unit of the input). F_out is the conversion factor of the output unit to watts. Dividing F_in by F_out gives the ratio needed to convert directly between any two units.
Source: NIST Special Publication 811 (2008 Edition) — Guide for the Use of the International System of Units (SI). Bureau International des Poids et Mesures (BIPM).
How it works
Power is the rate at which energy is transferred or work is performed over time. The SI unit of power is the watt (W), defined as one joule per second. All other power units can be expressed as fixed multiples or fractions of the watt, which makes unit conversion straightforward: multiply the input value by its conversion factor (in watts) and then divide by the target unit's conversion factor.
The conversion formula is P_out = P_in × (F_in / F_out), where F_in is the number of watts equivalent to one unit of the input and F_out is the number of watts equivalent to one unit of the output. Key conversion factors include: 1 mechanical horsepower = 745.69987 W, 1 metric horsepower (PS) = 735.49875 W, 1 BTU/hr = 0.29307107 W, and 1 foot-pound per second = 1.3558179 W. These exact values are standardised by NIST and the BIPM.
Power conversions arise constantly in practical engineering. Electrical engineers work in watts and kilowatts; automotive engineers use horsepower or PS; HVAC professionals rely on BTU/hr for heating and cooling loads; and mechanical engineers may encounter foot-pounds per second in legacy calculations. Understanding how these units relate helps you read datasheets, compare products, and comply with regional standards.
Worked example
Suppose you have a car engine rated at 150 mechanical horsepower (hp) and you want to know its power in kilowatts for a European specification sheet.
Step 1 — Convert to watts:
150 hp × 745.69987 W/hp = 111,854.98 W
Step 2 — Convert watts to kilowatts:
111,854.98 W ÷ 1,000 = 111.855 kW
So a 150 hp engine produces approximately 111.9 kW. You can also check the BTU/hr output:
111,854.98 W ÷ 0.29307107 W/(BTU/hr) = 381,640 BTU/hr — a figure relevant if you were estimating the cooling load on an engine room.
Limitations & notes
This converter uses internationally standardised conversion factors from NIST SP 811 and is accurate for all practical engineering purposes. However, a few caveats apply. The term "horsepower" is ambiguous: mechanical (imperial) horsepower (745.69987 W) is standard in the United States for automotive ratings, while metric horsepower (PS or CV) (735.49875 W) is used in Europe and Japan. Confusing the two introduces a ~1.4% error — small but meaningful in precision engineering. Electrical horsepower (746 W exactly) exists for motor ratings and is not included here. BTU/hr is a thermal power unit and is most accurate when applied to steady-state heat transfer; transient or peak power figures may need additional correction. Finally, this tool does not handle power density (W/m²), luminous efficacy (lm/W), or acoustic power — these require specialised converters.
Frequently asked questions
What is the exact conversion from horsepower to kilowatts?
For mechanical (imperial) horsepower, 1 hp = 745.69987 W = 0.74569987 kW. For metric horsepower (PS or CV), 1 PS = 735.49875 W = 0.73549875 kW. The difference is about 1.4%, so always clarify which standard applies when comparing specifications across regions.
Why does Europe use PS instead of hp for car engines?
PS stands for Pferdestärke (German for horsepower) and is the metric version defined as the power needed to raise 75 kilograms by 1 metre per second against gravity (9.80665 m/s²), giving exactly 735.49875 W. Imperial mechanical horsepower was defined differently by James Watt based on observed work rates of draft horses. The EU formally replaced PS with kilowatts in vehicle documentation but many consumers and manufacturers still cite PS informally.
How many watts are in 1 BTU per hour?
Exactly 1 BTU/hr = 0.29307107 W, or equivalently 1 W = 3.41214 BTU/hr. This conversion is used constantly in HVAC engineering — for example, a 10,000 BTU/hr air conditioner has a cooling capacity of about 2,931 W or 2.93 kW.
What is the difference between power and energy, and why does it matter for unit conversion?
Power is the rate of energy transfer (energy per unit time), while energy is the total amount transferred over a period. Watts measure power; watt-hours or joules measure energy. A 100 W light bulb consuming power for 10 hours uses 1,000 Wh = 1 kWh of energy. Mixing up power and energy units is a common error — always confirm whether a specification refers to instantaneous power (W) or cumulative energy (Wh, BTU, kJ).
When would an engineer use foot-pounds per second instead of watts?
Foot-pounds per second (ft·lbf/s) appears mainly in legacy American mechanical engineering calculations and some fluid mechanics references that predate SI standardisation. One mechanical horsepower is defined as exactly 550 ft·lbf/s. Modern engineering practice almost universally uses watts or kilowatts, but you may encounter ft·lbf/s in older textbooks, pump curves, or US industrial datasheets that haven't been updated to SI units.
Last updated: 2025-01-15 · Formula verified against primary sources.