Power Conversion Calculator

Convert between watts, kilowatts, megawatts, mechanical horsepower, metric horsepower, and BTU/hr instantly. Uses NIST and BIPM-standard constants for electrical, automotive, and HVAC work.

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Calculation Examples

Calculation Case Result
Electric motor: 5 kW to mechanical horsepower 6.705 hp
Car engine: 150 hp to kilowatts 111.855 kW
Air conditioner: 12,000 BTU/hr to watts 3,516.85 W

How to Use the Power Conversion Calculator

Enter the power value you want to convert, select the source unit from the "From" dropdown, then choose the target unit. The result appears immediately.

The calculator covers the full range of power units used across different engineering disciplines: SI units (W, kW, MW, GW) for electrical and utility work; mechanical and metric horsepower for automotive and industrial machinery; BTU/hr and tons of refrigeration for HVAC and thermal engineering. All conversions pass through the watt as the SI base unit: $$P_{\text{target}} = P_{\text{source}} \times \text{Conversion Factor}$$ One important distinction before you start: power is the rate of energy transfer (watts = joules per second), while energy is power multiplied by time (kilowatt-hours = kW × hours). This calculator converts power units only. If you need to convert energy, use a dedicated energy converter.

Technical Standards in Power Unit Conversions

Power is formally defined as the rate at which work is performed or energy is transferred. The SI unit is the Watt (W), defined as one joule per second: \[1\text{ W} = 1\text{ J/s} = 1\text{ V} \cdot \text{A}\] All conversion factors in this calculator are maintained by NIST and BIPM. The key constants are:

  • 1 mechanical horsepower (hp) = 550 ft·lbf/s = 745.69987 W exactly (by NIST definition)
  • 1 metric horsepower (PS) = 75 kgf·m/s = 735.49875 W exactly
  • 1 BTU/hr = 0.29307107 W (International Table BTU)
  • 1 ton of refrigeration (TR) = 3,516.852 W = 12,000 BTU/hr

The 1.4% difference between mechanical hp (745.7 W) and metric PS (735.5 W) is the most common source of error in automotive power comparisons. A European car rated at 200 PS produces 147.1 kW; the same figure in U.S. mechanical horsepower would be 200 hp = 149.1 kW. That 2 kW gap matters when sizing drive components or comparing dyno results across markets. The calculator distinguishes all horsepower variants explicitly so you pick the right one for your context.

Power unit conversion chart showing watts, kilowatts, mechanical horsepower, metric horsepower, and BTU/hr with conversion factors

Useful Tips 💡

  • Always confirm whether a horsepower rating is mechanical (hp), metric (PS), or electric (hp(E)). They differ by up to 1.4% and are not interchangeable in engineering specifications.
  • Power is a rate; multiplying it by time gives energy. \(1\text{ kW} \times 1\text{ h} = 1\text{ kWh}\). If your goal is to calculate energy consumption or cost, you need the time component as well.
  • For HVAC work, 1 ton of refrigeration = 12,000 BTU/hr = 3,516.852 W. This is the standard benchmark for sizing residential and commercial cooling systems in the U.S.

📋Steps to Calculate

  1. Enter the numerical power value into the input field.

  2. Select the source unit from the "From" dropdown (e.g., hp, kW, BTU/hr).

  3. Select the target unit from the "To" dropdown.

  4. Read the converted result instantly. For horsepower, confirm whether you need mechanical (hp), metric (PS), or electric (hp(E)) before selecting.

Mistakes to Avoid ⚠️

  1. Mixing mechanical horsepower (745.7 W) with metric horsepower/PS (735.5 W): a 200 PS European engine is not the same as 200 hp in U.S. specs. The difference is about 10 W per unit, or 2 kW at 200 units.
  2. Confusing power (watts) with energy (watt-hours): a 2,000 W heater running for 3 hours consumes 6 kWh of energy, not 6 kW.
  3. Assuming 1 BTU/hr equals approximately 1 W: the actual factor is 0.293 W per BTU/hr, so 1 W is 3.41 BTU/hr. The inverse is equally important for HVAC load calculations.
  4. Using rounded conversion factors in critical engineering work: the precise mechanical horsepower is 745.69987 W, not 746 W. At 500 hp, rounding to 746 W overstates the power by 1.5 kW.

Practical Applications📊

  1. Electrical engineering: Convert watts to kilowatts for circuit load balancing, energy billing calculations, and solar panel output assessments.

  2. Automotive industry: Translate engine output between kW and mechanical horsepower, or compare European PS ratings with U.S. hp specs on the same platform.

  3. HVAC systems: Convert BTU/hr to kilowatts and tons of refrigeration to size cooling equipment and verify energy efficiency ratings (EER, COP).

  4. Industrial machinery: Match motor nameplate ratings (often in hp) to electrical supply specifications (in kW) when sourcing equipment across different regional markets.

Questions and Answers

What is a power conversion calculator and why is it essential?

A power conversion calculator translates the rate of energy transfer between different unit systems: SI electrical units (watts, kilowatts), mechanical units (horsepower), and thermal units (BTU/hr, tons of refrigeration). These systems evolved independently across industries and regions, so equipment specs, safety standards, and billing data routinely use different units for the same physical quantity. Without accurate conversion, a motor sized in horsepower cannot be reliably matched to a switchboard rated in kilowatts, and an HVAC load calculated in BTU/hr cannot be compared to a utility bill in kWh without introducing systematic error.

How do I convert watts to kilowatts accurately?

Divide by 1,000: \(P_{(\text{kW})} = P_{(\text{W})} \div 1{,}000\). This is an exact metric prefix conversion (\(\text{kilo} = 10^3\)). For example, a 2,500 W electric oven draws 2.5 kW. The reverse (kW to W) multiplies by 1,000. This conversion is the most frequent in energy billing: utility meters record consumption in kWh, so knowing your appliance wattage and dividing by 1,000 gives the kilowatt figure needed to calculate running cost per hour.

What is the difference between mechanical and metric horsepower?

Mechanical horsepower (hp) is defined as 550 foot-pounds of force per second, equal to 745.69987 W by NIST. Metric horsepower (PS, from the German Pferdestärke) is defined as the power to raise 75 kilograms by 1 meter in 1 second, equal to 735.49875 W. The two differ by about 10.2 W per unit (1.4%). In practice: U.S. automotive specs use mechanical hp; European and Japanese specs historically used PS, though kW is now the EU legal standard for vehicle registration. A car rated at 300 PS produces 220.6 kW; the same car in U.S. hp would be 296.5 hp.

How do you convert horsepower to watts for motors?

Multiply mechanical horsepower by 745.69987: \(\text{W} = \text{hp} \times 745.69987\). For example, a 10 hp pump motor: \(10 \times 745.69987 = 7{,}456.99\) W = 7.457 kW. This figure determines the minimum circuit breaker rating and wire gauge for the installation. The National Electrical Code (NEC) uses this factor in Table 430.248 for motor full-load current calculations. For metric horsepower, use 735.49875 W instead.

What is the conversion factor from BTU/hr to watts?

One BTU/hr equals 0.29307107 W (International Table BTU, as defined by NIST). The inverse is 1 W = 3.41214 BTU/hr. A standard 12,000 BTU/hr (1-ton) air conditioner draws 3,516.85 W of cooling capacity. This conversion is central to HVAC energy efficiency ratings: EER (Energy Efficiency Ratio) = BTU/hr ÷ watts, so a 12,000 BTU/hr unit consuming 1,200 W has an EER of 10. Converting both figures to watts or both to BTU/hr first avoids the mixed-unit error that inflates or deflates efficiency calculations.

Is the watt an SI unit and how is it defined?

Yes. The watt is the SI unit of power, named after James Watt. It is defined as one joule of energy transferred per second: \(1\text{ W} = 1\text{ J/s}\). In electrical terms, \(1\text{ W} = 1\text{ V} \times 1\text{ A}\), making it directly measurable with standard instruments. The watt was adopted as an SI unit in 1960 and its definition has remained stable since, anchored to the joule, which in turn traces to the SI base units of kilogram, meter, and second. All other power units — horsepower, BTU/hr, ton of refrigeration — are defined by fixed conversion factors to the watt.

Which international standards does this power converter follow?

The calculator implements conversion constants from NIST Special Publication 811 (Guide for the Use of SI) and BIPM definitions. Mechanical horsepower (745.69987 W) and metric horsepower (735.49875 W) use the NIST-defined values. The BTU/hr factor (0.29307107 W) uses the International Table BTU, consistent with ASHRAE and ISO 31 thermal standards. These are the values used in regulatory submissions, ISO-certified test reports, and engineering standards such as ASME, IEC, and NEC.

What are the most common power conversion factors for engineering units?

Reference table of power units and their watt equivalents, from micro-electronics to heavy industry and thermal systems:

UnitSymbolValue in Watts (W)Scientific Notation
PicowattpW0.000000000001 W\(10^{-12}\) W
NanowattnW0.000000001 W\(10^{-9}\) W
MicrowattµW0.000001 W\(10^{-6}\) W
MilliwattmW0.001 W\(10^{-3}\) W
KilowattkW1,000 W\(10^{3}\) W
MegawattMW1,000,000 W\(10^{6}\) W
GigawattGW1,000,000,000 W\(10^{9}\) W
TerawattTW1,000,000,000,000 W\(10^{12}\) W
PetawattPW1,000,000,000,000,000 W\(10^{15}\) W
BTU per hourBTU/hr0.29307107 W—
Mechanical Horsepowerhp745.69987 W—
Metric HorsepowerPS735.49875 W—
Electric Horsepowerhp(E)746 W—
Boiler Horsepowerhp(S)9,812.5 W—
Ton of RefrigerationTR3,516.852 W—

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Disclaimer: This calculator is designed to provide helpful estimates for informational purposes. While we strive for accuracy, financial (or medical) results can vary based on local laws and individual circumstances. We recommend consulting with a professional advisor for critical decisions.