Watt (W)
The SI unit of power, equal to one joule per second; it measures the rate at which energy is produced, transferred or used.
The watt, named after James Watt, measures power: energy per unit time. It is not a quantity of energy; for that, multiply by time to obtain joules or watt-hours. Confusing the two is the most common error in energy reporting (see power vs energy).
1 kW = 10³ W; 1 MW = 10⁶ W; 1 GW = 10⁹ W; 1 TW = 10¹² W
1 horsepower (mechanical) = 745.7 W; 1 metric hp (PS) = 735.5 W
1 W = 3.412 BTU/h; 1 ton of refrigeration = 12,000 BTU/h = 3.517 kW
Indicative scales: an LED bulb 5–10 W; a human at rest about 100 W; a kettle 2–3 kW; a residential rooftop PV system 3–10 kWp; a modern onshore wind turbine 4–7 MW; offshore turbines 12–15 MW and larger; a large nuclear reactor about 1–1.6 GW electric; global average electricity demand about 3.6 TW (31,772 TWh / 8,760 h in 2025).
Suffixes clarify what is measured: MWe (electric output), MWth (thermal), Wp or kWp (PV peak power under STC), and VA or var for apparent and reactive power in AC circuits. Installed capacity in watts says nothing about annual output, which depends on the capacity factor.
Related terms
All terms →- Kilowatt-hour (kWh)
A unit of energy equal to one kilowatt of power sustained for one hour, or 3.6 megajoules; the standard billing unit for electricity.
- Power vs energy (kW vs kWh)
Power (kW) is the rate of energy flow at an instant; energy (kWh) is power accumulated over time. Capacity is measured in power, output in energy.
- Joule (J)
The SI unit of energy, work and heat: the work done by a force of one newton over one metre, or one watt sustained for one second.
- Kilowatt-peak (kWp)
The DC power rating of a PV module or array under Standard Test Conditions; the conventional unit of installed solar capacity.
- Capacity factor
The ratio of the energy a power plant actually produces over a period to the energy it would produce running at full nameplate capacity for the whole period.