What Is a Watt?
A watt (W) is the International System of Units (SI) unit of power. One watt represents an energy transfer of one joule per second (1 W = 1 J/s = 1 kg × m² / s³). Power describes the rate at which energy is transferred or converted, so watts are used to measure everything from tiny electronic circuits to massive power plants.
How to Convert Watts to kW and mW
Converting between watts and other SI prefixed units is straightforward — just multiply or divide by the appropriate power of ten:
- To convert watts → kilowatts: divide by 1,000 (1 kW = 10³ W)
- To convert kilowatts → watts: multiply by 1,000
- To convert watts → milliwatts: multiply by 1,000 (1 mW = 10⁻³ W)
- To convert milliwatts → watts: divide by 1,000
SI Power Unit Reference Table
| Name | Symbol | Conversion Factor |
|---|---|---|
| Picowatt | pW | 10⁻¹² W |
| Nanowatt | nW | 10⁻⁹ W |
| Microwatt | µW | 10⁻⁶ W |
| Milliwatt | mW | 10⁻³ W |
| Watt | W | 1 W |
| Kilowatt | kW | 10³ W |
| Megawatt | MW | 10⁶ W |
| Gigawatt | GW | 10⁹ W |
| Terawatt | TW | 10¹² W |
Other Power Units
SI prefixes are not the only way to express power. Depending on the context, you may encounter:
- Mechanical horsepower (hp) — 1 hp = 745.7 W. Common in the United States for engine ratings.
- Metric horsepower (PS) — 1 PS = 735.5 W. Used in Europe for vehicle engine ratings.
- BTU per hour (BTU/h) — 1 BTU/h ≈ 0.293 W. Standard in HVAC systems in the US.
- Foot-pound per second (ft·lbf/s) — 1 ft·lbf/s ≈ 1.356 W. Used in mechanical engineering.
- Erg per second (erg/s) — 1 erg/s = 10⁻⁷ W. Used in the CGS system.
- Calorie per second (cal/s) — 1 cal/s = 4.187 W.
- Kilocalorie per hour (kcal/h) — 1 kcal/h ≈ 1.163 W. Common in nutrition and HVAC contexts.
How to Convert Horsepower to Watts
To convert mechanical horsepower to watts, multiply by 745.69987:
Power (W) = HP × 745.69987
For metric horsepower (PS), multiply by 735.49875:
Power (W) = PS × 735.49875
Conversion Examples
- 1 hp → watts: 1 × 745.7 = 745.7 W
- 100 W → kW: 100 ÷ 1000 = 0.1 kW
- 1 kW → hp: 1000 ÷ 745.7 ≈ 1.341 hp
- 5000 BTU/h → W: 5000 × 0.2931 ≈ 1465.5 W
- 1 MW → GW: 1 ÷ 1000 = 0.001 GW
FAQs
What is the difference between watts and kilowatts?
A kilowatt equals 1,000 watts. Kilowatts are commonly used to rate home appliances and small engines, while individual watts are more common for electronics and lighting.
What is the difference between mechanical and metric horsepower?
Mechanical (imperial) horsepower (hp) = 745.7 W and is standard in the US. Metric horsepower (PS or CV) = 735.5 W and is used in Europe. The difference is about 1.4%.
How many watts does a typical home use?
An average US household uses about 1.2 kW (1,200 W) continuously, or roughly 10,500 kWh per year. Peak demand can be 3–5 kW or more when multiple high-power appliances are running simultaneously.
What is a gigawatt?
One gigawatt (GW) = 1,000,000,000 W = 1,000 MW. Large power plants typically generate 1–3 GW. The total electricity capacity of the United States is over 1,000 GW.
How do I convert erg/s to watts?
The erg per second belongs to the CGS (centimetre–gram–second) unit system. 1 erg/s = 10⁻⁷ W, or equivalently 10,000,000 erg/s = 1 W.