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The Power Conversion Calculator converts between all major units of power used in physics, engineering, and industry. Power is the rate of energy transfer or conversion per unit time: $$P = \frac{dE}{dt}$$ The SI unit is the watt (W), equal to one joule per second. Enter any power value and select its unit to see instant conversions to watts, kilowatts, megawatts, horsepower, BTU/h, and foot-pounds per second.
Power measurements span an enormous range — from nanowatts in electronic sensors to terawatts for global energy consumption. Engineers routinely work across multiple unit systems: electrical power in kilowatts, engine output in horsepower, heating/cooling capacity in BTU/h, and scientific calculations in watts. This calculator bridges all these conventions with precise conversion factors.
All conversions use the watt as the intermediate reference unit. The input is first converted to watts, then from watts to every other unit. The conversion factors are:
$$1\;\text{kW} = 1000\;\text{W}$$ (exact)
$$1\;\text{MW} = 10^6\;\text{W}$$ (exact)
$$1\;\text{HP} = 745.69987\;\text{W}$$ (mechanical/Imperial horsepower)
$$1\;\text{BTU/h} = 0.29307107\;\text{W}$$
$$1\;\text{ft·lb/s} = 1.3558179\;\text{W}$$
Background on each unit:
The relationship between power and energy is fundamental: $$E = P \times t$$. A 100 W light bulb running for 10 hours consumes 1 kWh = 3.6 MJ of energy. Power ratings on appliances indicate instantaneous energy consumption rate, not total energy used.
For rotating machinery, power relates to torque and angular velocity by $$P = \tau\omega$$, where $$\tau$$ is torque in N·m and $$\omega$$ is angular velocity in rad/s. This is why engine specifications always list both horsepower and torque — they provide complementary information about performance.
All six output values represent the same physical rate of energy transfer in different units. Use watts/kilowatts for electrical and scientific applications, horsepower for engine and motor ratings, BTU/h for HVAC systems, and ft·lb/s for mechanical engineering in Imperial units. The conversions are precise to 6+ significant figures for all units.
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A 200 HP car engine produces about 149 kW of mechanical power. At full throttle, it consumes roughly 509,000 BTU/h of thermal energy from fuel (before accounting for engine efficiency of ~25–30%).
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A 2-ton residential AC unit has a cooling capacity of 24,000 BTU/h = 7.03 kW. The electrical power drawn is typically 2–3 kW (the rest comes from moving heat rather than creating it, via the refrigeration cycle).
Mechanical (Imperial) horsepower = 745.70 W, defined as 550 ft·lb/s by James Watt. Metric horsepower (PS, from German Pferdestärke) = 735.50 W, defined as the power to lift 75 kg by 1 m in 1 s. Electrical horsepower = 746 W exactly. This calculator uses mechanical HP, the most common in US/UK engineering.
One ton of cooling = 12,000 BTU/h = 3.517 kW. It originates from the rate at which one ton (2000 lb) of ice at 32°F melts in 24 hours. Residential AC units are typically 1.5–5 tons. A ton of cooling is a measure of heat removal rate, not weight.
Watts measure instantaneous power; electricity is billed in kilowatt-hours (energy). A 1500 W space heater running for 8 hours uses 12 kWh. At $0.12/kWh, that costs $1.44. Power (W) × time (h) = energy (Wh), and energy × rate = cost.
Watt needed a marketing metric to sell his improved steam engines. He measured that a mill horse could sustain about 550 ft·lb/s (turning a mill wheel), and defined this as 1 HP. He may have deliberately overestimated horse power so his engines would always exceed their HP rating — good for sales.
For rotating machinery: $$P = \tau \omega = \frac{2\pi n \tau}{60}$$ where $$\tau$$ is torque (N·m), $$\omega$$ is angular velocity (rad/s), and $$n$$ is RPM. In Imperial: HP = torque(ft·lb) × RPM / 5252. HP and torque curves always cross at 5252 RPM.
At rest, the human body produces about 80–100 W of thermal power (basal metabolic rate). During intense exercise, a fit person can sustain ~200–400 W of mechanical output, with elite cyclists exceeding 400 W for extended periods. Peak output (sprinting) can briefly reach ~2000 W.
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