The problem: power is a rate, not an amount
Power and energy are not the same thing, and the difference matters every time you look at an electricity bill.
Energy is an amount — the total quantity of work done or heat transferred. The SI unit is the joule. Lifting a 1-kilogram mass 1 meter against gravity takes about 9.81 joules. A 100-watt light bulb running for 10 seconds uses 1,000 joules.
Power is the rate at which energy is transferred — energy divided by time. The SI unit is the watt, defined as one joule per second. A 100-watt light bulb uses 100 joules every second. A 1,000-watt microwave uses 1,000 joules every second. The microwave and the bulb running for the same duration use the same total energy if their power is the same; the microwave just uses it ten times faster.
The unit that confuses everyone is the kilowatt-hour, which appears on electricity bills. A kilowatt-hour is not a unit of power. It is a unit of energy — 1,000 watts sustained for one hour, which is 1,000 joules/second × 3,600 seconds = 3.6 million joules. If your electricity costs $0.15 per kWh, you are paying for energy, not power. The power is what the appliance draws (watts); the energy is what you are billed for (watt-hours).
The Power Unit Converter converts between 9 power units across three systems — metric (W, kW, MW, J/s), mechanical (horsepower mechanical, horsepower metric, foot-pound per second), and thermal (BTU per hour, calorie per second). It does not convert energy units; for that, use a separate energy converter.
Fastest path
Open the Power Unit Converter, enter your value, pick the source unit and target unit from the dropdowns. The result appears immediately, with the formula 1 [from] = [factor] [to] shown below. The All Units table shows your input converted to all 9 units simultaneously, grouped by category. Use the Swap button to reverse direction. Click any row in the All Units table to set it as the target. Adjust precision (2 to 12 decimal places) for very small or very large factors. The preset buttons load common reference points: a 60W light bulb, 1 horsepower, a 100 kW engine, a 1 MW power plant, a 10,000 BTU/h air conditioner, a 500 cal/s burner.
The watt: one joule per second
The watt is the SI derived unit of power, defined as one joule per second: 1 W = 1 J/s. This is not a coincidence — the SI system is coherent, meaning the derived units are products of base units with no numerical factors. One watt is exactly one joule per second, one newton-meter per second, one kilogram-meter-squared per second cubed. All of these are the same unit written different ways for different contexts.
The metric prefixes scale the watt by powers of 1,000:
| Unit | Symbol | Value | Typical use |
|---|---|---|---|
| Milliwatt | mW | 0.001 W | Laser pointers, small electronics |
| Watt | W | 1 W | Light bulbs, small appliances |
| Kilowatt | kW | 1,000 W | Microwaves, heaters, car engines |
| Megawatt | MW | 1,000,000 W | Building power, small generators |
| Gigawatt | GW | 1,000,000,000 W | Power plants |
The tool includes W, kW, and MW (plus J/s, which is identical to W). It does not include GW, mW, or smaller prefixes. For most engineering and household conversions, the included range is sufficient — a 1 GW power plant converts to 1,000 MW, which the tool handles by setting the input unit to MW.
Horsepower: two different definitions
Horsepower is where power conversion gets political. There are two common definitions, and they differ by about 1.4 percent.
Mechanical horsepower (also called imperial horsepower, hp(I)): 745.6998716 W. Used in the US and UK. Defined by James Watt in the late 18th century as 33,000 foot-pounds-force per minute — the rate at which a draft horse could lift coal from a mine. Watt was selling steam engines and needed a unit customers could relate to. He estimated a horse could lift 33,000 pounds by 1 foot every minute, which is 550 foot-pounds per second, which converts to 745.7 watts.
Metric horsepower (also called Pferdestärke, PS, hp(M)): 735.49875 W. Used in most of Europe and countries that adopted the metric system. Defined as the power to lift 75 kilograms by 1 meter in 1 second: 75 kg × 9.80665 m/s² × 1 m/s = 735.5 W. The metric definition is cleaner (round numbers in metric units) but produces a value 1.4 percent lower than the mechanical definition.
A car rated at 100 hp in the US produces 74.57 kW. The same car rated at 100 PS (metric horsepower) in Germany produces 73.55 kW. The numbers look the same (100) but the power is different. European car magazines sometimes quote both, and the conversion is not symmetric — a 100 PS engine is a 98.6 hp engine, not a 100 hp engine.
The tool includes both, labeled hp(I) and hp(M). Always check which horsepower a specification uses. US and UK specs are mechanical; European and Japanese specs (when quoted in PS rather than kW) are metric. Marketing materials sometimes use whichever makes the number larger.
BTU per hour: the HVAC unit
The British Thermal Unit (BTU) is a unit of energy — the heat required to raise 1 pound of water by 1 degree Fahrenheit, about 1,055 joules. BTU per hour is a unit of power: energy per time. One BTU per hour is 0.293071 W.
BTU/h is the standard unit for heating and cooling equipment in the US, even as the rest of the world uses watts. An air conditioner rated at 10,000 BTU/h is drawing about 2,930 W of cooling capacity (not electrical input — that depends on the coefficient of performance). A residential furnace might be 80,000 BTU/h (about 23.4 kW). A commercial boiler might be 500,000 BTU/h (about 146.5 kW).
The tool includes BTU/h in the Thermal category. The preset button "10,000 BTU/h AC" loads a typical window-unit air conditioner — a useful reference point for understanding the scale. The conversion 10,000 BTU/h = 2.93 kW shows that a window AC draws roughly the same power as a microwave oven or a small space heater, which is why a dedicated circuit is recommended.
Calorie per second and foot-pound per second
The remaining units are less common but appear in specific fields:
Calorie per second (cal/s): 4.1868 W. The thermochemical calorie is the energy to raise 1 gram of water by 1 degree Celsius. The "Calorie" on nutrition labels is actually a kilocalorie (1,000 calories). Cal/s appears in thermodynamics problems, metabolic-rate calculations, and heating-cooling load analysis. A resting human metabolic rate of about 100 W is about 24 cal/s. The tool's preset "500 cal/s Burner" represents a typical gas-stove burner on high — about 2,093 W or 2.1 kW.
Foot-pound-force per second (ft-lbf/s): 1.3558179 W. The imperial mechanical unit, related to mechanical horsepower by a factor of 550 — 1 mechanical hp = 550 ft-lbf/s. This unit appears in older engineering texts and in some US mechanical engineering contexts. A 1-hp motor produces 550 ft-lbf/s of work.
Joules per second: the identity
The tool includes J/s as a separate unit, but it is identical to the watt by definition. 1 J/s = 1 W. The unit exists in the tool for users who encounter it in physics or engineering texts and want to convert to more practical units. Converting 1 J/s to W gives 1.000000 — they are the same thing.
This identity is the reason the SI system uses the watt as the named power unit rather than joule-per-second. Both are correct; the watt is shorter and is the standard name. When you see J/s in a textbook, read it as "watts."
Real power vs apparent power: the missing unit
The tool does not include volt-amperes (VA), which is an important distinction in AC power. In a DC circuit, power is voltage times current: P = V × I, in watts. In an AC circuit driving a resistive load (a heater, an incandescent bulb), the same formula holds, and watts are watts.
But with reactive loads (motors, transformers, anything with coils or capacitors), the current and voltage are not in phase. Some of the current flows back and forth without doing useful work. The apparent power — the product of RMS voltage and RMS current — is measured in volt-amperes (VA). The real power — what the load actually consumes — is measured in watts. The ratio is the power factor: P_real = P_apparent × power_factor.
A 1,000-VA generator with a 0.8 power factor delivers 800 W of real power. A motor with a 0.75 power factor draws 1,333 VA to produce 1,000 W. Utilities care about VA because their wires and transformers must carry the apparent current, not just the real power. Industrial customers are sometimes billed on kVA (apparent) rather than kW (real) to account for this.
The tool's omission of VA is a simplification. For most household and light-commercial conversions, the power factor is close to 1 (resistive loads) and the distinction does not matter. For industrial equipment sizing, the difference between W and VA can be 20 to 40 percent and matters significantly. If you are sizing a UPS or a generator for a motor load, use a tool that includes power factor.
The horsepower variants the tool does not include
Beyond mechanical and metric horsepower, there are several specialized definitions:
Electrical horsepower: 746 W exactly. Used in the US for rating electric motors. The value is rounded to a clean number for convenience. It differs from mechanical horsepower (745.7 W) by 0.04 percent — negligible for most purposes but real for precision work.
Boiler horsepower: 9,809.5 W. Used to rate steam boilers, defined as the power to evaporate 34.5 pounds of water per hour at 212°F. This is about 13 times the mechanical horsepower — a 100-boiler-hp steam plant is not the same as a 100-hp engine.
Hydraulic horsepower: the power transmitted by a hydraulic fluid, calculated from pressure × flow. It equals mechanical horsepower for the same numerical value but is used in fluid-power engineering.
Refrigeration ton: 3,516.85 W. Not a horsepower variant but a power unit used in HVAC. One ton of refrigeration is the power to melt 1 short ton of ice in 24 hours — about 3.52 kW. A 4-ton air conditioner is about 14.1 kW, roughly 48,000 BTU/h.
The tool omits these because they are niche. Mechanical and metric horsepower cover most engine and motor ratings. BTU/h covers HVAC. The specialized variants appear in specific industries and are rarely mixed with general power conversions.
Gotchas
- Power is a rate, energy is an amount. The watt is joules per second. The kilowatt-hour is 3.6 million joules. If you are converting a power rating (engine output, appliance draw, plant capacity), use the Power Converter. If you are converting an energy quantity (battery capacity in kWh, fuel energy in BTU, heating requirement in joules), use an Energy Converter. Mixing the two produces off-by-3,600 errors.
- Mechanical and metric horsepower are not the same. Mechanical hp is 745.7 W; metric hp (PS) is 735.5 W — 1.4 percent less. A 100-hp US car and a 100-PS German car produce different power. Always check which definition a specification uses, especially when comparing vehicles across regions.
- BTU/h is not BTU. BTU is energy (about 1,055 J). BTU/h is power (about 0.293 W). An air conditioner rated at 10,000 BTU/h has a cooling power of 2.93 kW. If a spec says "10,000 BTU" without the /h, it is likely shorthand for BTU/h in an HVAC context, but check — 10,000 BTU as an energy quantity is about 2.93 Wh, which is a tiny amount.
- The tool omits volt-amperes (VA) and power factor. For resistive loads (heaters, incandescent lights, electronics with power-factor correction), VA equals W and the omission does not matter. For motors, transformers, and fluorescent lights without correction, apparent power (VA) can exceed real power (W) by 20 to 40 percent. If you are sizing a generator, UPS, or circuit for a reactive load, use a tool that includes power factor.
- Boiler and electrical horsepower are different from mechanical. Boiler hp is 9,809 W (about 13x mechanical). Electrical hp is 746 W exactly (essentially the same as mechanical for practical use). If a spec says "horsepower" in a boiler or electric-motor context, verify which definition applies — the numbers are not interchangeable.
Summary
- Power is a rate (energy per time); the SI unit is the watt, defined as one joule per second. Energy is an amount; the kilowatt-hour on an electricity bill is 3.6 million joules. Use the Power Converter for rates (W, kW, hp, BTU/h) and an Energy Converter for quantities (J, kWh, BTU).
- The metric prefixes scale the watt by 1,000: mW, W, kW, MW, GW. The tool includes W, kW, MW, and J/s (identical to W). Horsepower has two definitions: mechanical hp(I) = 745.7 W (US/UK), metric hp(M) = 735.5 W (Europe), differing by 1.4 percent.
- BTU per hour (0.293 W) is the standard HVAC unit in the US. A 10,000 BTU/h air conditioner draws about 2.93 kW. Calorie per second (4.187 W) appears in thermodynamics and metabolic-rate calculations. Foot-pound per second (1.356 W) is the imperial mechanical unit, related to mechanical horsepower by a factor of 550.
- The tool does not include volt-amperes (VA) or power factor. For resistive loads, VA equals W. For reactive loads (motors, transformers), apparent power can exceed real power by 20 to 40 percent — use a tool that includes power factor for industrial sizing.
- Use the Power Unit Converter for power conversions, the Energy Converter for energy quantities, the Weight Converter for mass, and the Temperature Converter for temperature (which has offset, not just scaling).