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Power Conversion Calculator

Power measurement spans industrial, electrical, and mechanical domains—each with its own standard units. This calculator bridges nine SI-based watt denominations, five horsepower variants, BTU per hour, kilocalories per hour, ergs per second, and refrigeration tons. Enter any value and instantly see equivalents across all supported units, essential for HVAC technicians sizing cooling capacity, engineers comparing motor ratings, and professionals navigating mixed measurement systems.

Last updated: May 9, 2026

Creators Wojciech Sas, PhD

Reviewers Karolina Tórzala and Filip Derma

7,720 people find this calculator helpful

Understanding Power Units

Power quantifies the rate at which energy transfers or work is performed, measured in joules per second. The SI standard unit, the watt (W), underpins most modern applications, yet dozens of alternative units persist in specific industries.

Metric multiples: Kilowatts (kW), megawatts (MW), gigawatts (GW), and terawatts (TW) scale upward by thousands for industrial and grid-scale systems.
Submultiples: Milliwatts (mW), microwatts (µW), and nanowatts (nW) address delicate instrumentation and low-power electronics.
Horsepower variants: Mechanical horsepower (~745.7 W) and metric horsepower (~735.5 W) originated from 18th-century comparisons with draft animal capabilities and remain embedded in automotive and machinery specifications.
Thermal and refrigeration units: BTU per hour measures heating and cooling capacity, while tons of refrigeration (TR) quantifies industrial chiller output.

Power Conversion Formulas

All conversions reference the watt as the base unit. Here are the relationships for the most frequently used equivalencies:

1 mW = 0.001 W = 10⁻³ W

1 kW = 1,000 W = 10³ W

1 MW = 1,000,000 W = 10⁶ W

1 mechanical hp = 745.7 W

1 metric hp = 735.5 W

1 BTU/h ≈ 0.293 W

1 kcal/h ≈ 1.163 W

1 ton of refrigeration ≈ 3,516.85 W

W — Watts, the SI unit of power (joules per second)
mW — Milliwatts, one-thousandth of a watt
kW — Kilowatts, one thousand watts
MW — Megawatts, one million watts
hp — Horsepower, ranging from 735.5 W (metric) to 745.7 W (mechanical)
BTU/h — British thermal units per hour, common in HVAC applications
kcal/h — Kilocalories per hour, used in thermal engineering
TR — Tons of refrigeration, standard in industrial cooling systems

The Watt Explained

The watt represents one joule of energy transferred or dissipated per second. Mechanically, it equals the power needed to exert a force of one newton across a distance of one meter in one second. Electrically, one watt flows when one ampere of current moves across a one-volt potential difference:

Mechanical: 1 W = 1 N·m/s = 1 kg·m²/s³ in base SI units
Electrical: 1 W = 1 A × 1 V

This makes the watt universally applicable across mechanical, electrical, thermal, and chemical domains. A 60-watt incandescent bulb dissipates 60 joules of heat and light energy every second; a 2 kW kettle draws 2,000 joules per second when heating water.

Common Conversion Pitfalls

Real-world power conversions often reveal subtle but important distinctions.

Horsepower ambiguity across industries — Mechanical horsepower (745.7 W) and metric horsepower (735.5 W) differ by about 1.4%, yet legacy automotive specifications, diesel engine ratings, and European machinery labels may use either without explicit notation. Always verify the source region or manufacturer before relying on rounded 'hp = 746 W' approximations.
BTU/h misleads on electric vs. thermal systems — A 10,000 BTU/h air conditioner outputs cooling equivalent to ~2,931 watts, but does not consume 2,931 watts of electricity; actual input power depends on seasonal energy efficiency ratio (SEER). Confusing cooling capacity with electrical power draw is common in HVAC troubleshooting.
SI prefix scaling is exponential, not linear — Kilowatts to megawatts is 1,000× not 10×. Off-by-order-of-magnitude errors are frequent when mental math substitutes for calculation, especially in high-voltage grid contexts where gigawatts and terawatts routinely appear.
Historical horsepower definitions vary by context — Beyond mechanical and metric, boiler horsepower (~9.81 kW), electric horsepower (~746 W), and air horsepower (~745.7 W) each serve narrow specialisms. Older industrial equipment may reference obsolete local variants not covered by standard converters.

Practical Applications and Examples

Power conversions become essential in multiple scenarios:

HVAC sizing: A technician specifying a replacement compressor in BTU/h must convert to kilowatts to match electrical supply constraints and compare equipment efficiency ratings.
Motor selection: Industrial machinery nameplates list rated power in horsepower, yet grid interconnection requires kilowatts; a 10 hp motor (~7.46 kW) demands appropriate breaker and wiring gauge.
Energy audits: Consumption invoices report kilowatt-hours; converting appliance labels from watts clarifies monthly operating costs.
Renewable energy: Photovoltaic arrays rated in kilowatts, wind turbines in megawatts, and nuclear plants in gigawatts all feed a unified grid measured in watts per second.

Frequently Asked Questions

How do I convert horsepower to watts?

Multiply the horsepower value by the appropriate conversion factor. Mechanical horsepower (standard in North America) equals 745.7 watts, so 5 hp = 5 × 745.7 = 3,728.5 W. Metric horsepower (common in Europe) is 735.5 watts; 5 metric hp = 5 × 735.5 = 3,677.5 W. The ~1.4% difference arises from historical definitions—James Watt's mechanical unit was based on observed horse performance, while the metric variant adopted the standard kilogram-force.

What's the difference between BTU and BTU/h in power conversion?

BTU (British thermal unit) measures energy, not power. BTU/h (BTU per hour) measures power—the rate at which thermal energy is delivered. One BTU/h ≈ 0.293 watts. A 10,000 BTU/h air conditioner delivers 2,931 watts of cooling capacity. Conflating the two units is a frequent error; always check whether a specification refers to total energy released or rate of release per unit time.

Why does my electric motor label show horsepower but the breaker is rated in amperes?

Horsepower reflects mechanical output; amperes measure electrical input current. The relationship depends on voltage and efficiency. A 1 hp motor at 120 volts requires roughly 9.8 amperes (at full load), while the same motor at 240 volts draws ~4.9 amperes. The 746 watts output demands approximately 800–900 watts of electrical input (accounting for motor losses), which translates to different currents depending on supply voltage and power factor.

How many watts is a ton of refrigeration?

One ton of refrigeration (TR) equals approximately 3,516.85 watts, or 3.52 kW. This unit originated in the ice industry and represents the cooling power needed to melt one short ton (2,000 pounds) of ice in 24 hours. Industrial chiller specifications often list capacity in TR; a 100 TR unit provides ~351.7 kW of cooling output, useful for comparing capacity across equipment databases that mix metric and imperial notations.

Can I convert watts directly to kilocalories per hour?

Yes. One kilocalorie per hour (kcal/h) ≈ 1.163 watts. So 100 kcal/h ≈ 116.3 W. This conversion matters in dietetics, thermodynamics, and older engineering texts that predate SI standardization. For example, a 1 kW heating element dissipates roughly 860 kcal/h, a figure commonly cited in European thermal design.

Why do I need this calculator instead of a simple formula?

Power unit proliferation across industries means memorizing dozens of conversion factors is impractical. A single entry in this calculator displays simultaneous equivalencies across nine SI multiples, five horsepower variants, BTU/h, kilocalories, ergs, and refrigeration tons. This eliminates lookup errors, reduces unit algebra mistakes, and accelerates cross-domain communication—critical when engineers, technicians, and procurement specialists must align specifications using different standards.

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