Power is energy per unit of time
Power describes how quickly energy is transferred, converted, or used. A small amount of energy delivered very quickly can represent high power, while a large amount delivered slowly can represent modest power. This distinction appears in electrical equipment, engines, heating systems, power stations, batteries, and human performance.
The watt is the International System of Units measure of power. One watt equals one joule per second. Other fields commonly use kilowatts, megawatts, horsepower, Btu per hour, or kilocalories per hour.
This calculator converts every input to watts and then divides by the watt value of the target unit. That shared base keeps the factors traceable and avoids maintaining a separate table for every possible pair.
Watts, kilowatts, megawatts, and gigawatts
Metric prefixes scale the watt by exact powers of ten:
1 mW = 0.001 W
1 kW = 1,000 W
1 MW = 1,000,000 W
1 GW = 1,000,000,000 W
Milliwatts suit low-power electronics and radio signals. Kilowatts are common for household appliances, motors, solar systems, and vehicle output. Megawatts describe large generators and industrial loads. Gigawatts describe the scale of major power stations or whole electricity systems.
The symbol is case-sensitive. mW means milliwatt; MW means megawatt. Confusing them introduces a factor of one billion.
Mechanical and metric horsepower
Horsepower is not one universal unit. Mechanical horsepower, used widely for engines in the United States and United Kingdom, is approximately:
1 mechanical hp = 745.6998715822702 W
Metric horsepower, often written PS, cv, ch, or hk depending on language and market, is defined from 75 kilogram-force metres per second:
1 metric hp = 735.49875 W
The difference is about 1.39%. That is large enough to matter in product comparisons. A vehicle advertised as 200 PS produces about 197.26 mechanical hp, even though both numbers may be casually called horsepower.
Some industries have additional definitions, including electrical and boiler horsepower. This converter labels the two included definitions explicitly rather than guessing what an unlabeled value means.
Btu per hour
The British thermal unit is an energy unit. Dividing it by an hour creates a power unit used for heating and cooling rates.
Using the International Table Btu:
1 Btu/h = 0.2930710701722222 W
1 W ≈ 3.412141633 Btu/h
An air conditioner described as “12,000 BTU” usually means a cooling capacity of 12,000 Btu per hour, approximately 3.517 kW of thermal power. That is not the same as its electrical input. The ratio between delivered cooling and electrical consumption depends on efficiency and rating conditions.
Different historical Btu definitions vary slightly. The tool uses Btu International Table and states that convention so a small disagreement with another source can be explained.
Kilocalories per hour
The thermochemical calorie is exactly 4.184 joules, and a kilocalorie is 4,184 joules. Dividing by 3,600 seconds per hour gives:
1 kcal/h = 1.1622222222 W
Kilocalories per hour can appear in heat-transfer and metabolic contexts. A food Calorie is also a kilocalorie, but converting a person’s daily dietary energy into average watts is only an arithmetic comparison. Human metabolism varies over time and does not deliver all dietary energy as useful mechanical work.
Power versus energy
Power and energy answer different questions:
energy = power × time
power = energy ÷ time
A 2 kW heater running for three hours uses 6 kWh of energy. A battery storing 6 kWh could theoretically power it for three hours, but real runtime is lower because of conversion losses, reserve limits, and changing load.
A device rated at 100 W does not necessarily consume 100 Wh in every use. It consumes 100 Wh only if it draws that power continuously for one hour. Variable-speed appliances and electronics change their power during operation.
This calculator converts power units only. Use an energy converter for joules and kWh, and include time when moving between the two quantities.
Converting with a watt base
The general method is:
target = source × source watts per unit ÷ target watts per unit
To convert 50 kW to mechanical horsepower, multiply by 1,000 W/kW and divide by 745.6998716 W/hp. The result is approximately 67.0511 hp.
To convert 20,000 Btu/h to kW, multiply by 0.2930710702 W per Btu/h and divide by 1,000. The result is approximately 5.86142 kW.
To convert 150 metric horsepower to kW, multiply by 735.49875 W and divide by 1,000, producing approximately 110.3248 kW.
Input power, output power, and efficiency
A unit conversion does not account for efficiency. Motors, engines, inverters, chargers, pumps, and cooling systems lose some input energy or deliver it in forms other than the desired output.
output power = input power × efficiency
A motor drawing 10 kW electrically at 90% efficiency delivers about 9 kW mechanically. Converting 10 kW directly to horsepower would describe the electrical input as though it were shaft output and overstate performance.
Likewise, a heat pump may deliver more thermal power than its electrical input because it moves heat rather than converting electricity entirely into heat. Its coefficient of performance connects those two figures. They should not be compared as if they were alternative unit labels for one quantity.
Peak, average, and continuous ratings
The same equipment may have several valid power figures. Peak power is available briefly. Continuous power can be sustained under defined thermal conditions. Average power is energy divided by the measurement duration. Nameplate input may be a maximum rather than typical consumption.
A loudspeaker amplifier, solar inverter, electric motor, and vehicle can each advertise a peak number that is not comparable with another product’s continuous rating. Convert units only after confirming that the measurements represent the same type of rating and test conditions.
For variable loads, average power over a period is often more useful for energy cost. Short peaks still matter for cable sizing, inverters, protection devices, and thermal design.
Electrical power needs more information
Watts cannot always be calculated from voltage and current by simple multiplication. For direct current, power is voltage multiplied by current. For alternating current, phase, power factor, and waveform can matter.
Single-phase real power for a sinusoidal load is voltage × current × power factor. Balanced three-phase systems include a factor of the square root of three when line quantities are used. Apparent power in volt-amperes is not identical to real power in watts unless power factor is one.
This converter begins with an already known power value. It does not infer watts from electrical measurements.
Worked comparisons
A 1.5 kW motor rating equals about 2.0115 mechanical hp or 2.0394 metric hp. If 1.5 kW is electrical input rather than mechanical output, efficiency must be applied before comparing it with an engine’s shaft power.
A 3.5 kW thermal output equals about 11,942.5 Btu/h. That is close to the common 12,000 Btu/h air-conditioning class. The electrical draw may be around one kilowatt or another value depending on efficiency and conditions.
A 500 MW generator equals 0.5 GW and 500,000 kW. Expressing the same quantity at the appropriate scale improves readability without changing the physical power.
One mechanical horsepower equals about 641.616 kcal/h. That number represents an energy-transfer rate, not the food energy a person must consume to sustain one horsepower of mechanical output.
Precision and significant figures
Defined conversion factors may carry many digits, but a measured input rarely does. A motor rated 7.5 kW may not deliver exactly 7,500.000 watts under every condition. Showing a long converted value prevents intermediate rounding error but does not add measurement accuracy.
Round the final result to match the purpose and input precision. Product comparisons may need two or three significant figures; engineering calculations may retain more before a final tolerance analysis.
Do not round a factor early when converting very large totals or chaining calculations. Keep full precision internally, as this tool does, then format the reported answer.
Limits of this converter
The tool changes unit labels for one power quantity. It does not calculate energy, runtime, efficiency, electrical power factor, fuel consumption, cooling load, or equipment suitability.
It uses mechanical horsepower, metric horsepower, the International Table Btu, and the thermochemical kilocalorie. Another horsepower or Btu convention can produce a different result.
For equipment selection, safety, billing, certification, or regulatory work, verify whether the source value is input, output, peak, continuous, thermal, mechanical, or electrical power. A numerically correct conversion cannot repair a mismatch in what the two ratings mean.