Watt (W)

Definition

The SI unit of power, one joule per second.

History

The watt is the SI unit of power, one joule per second, named after James Watt for his work on the steam engine. It measures the rate at which energy is used or produced.

Where it is used

Watts rate light bulbs, appliances and solar panels the world over.

Typical uses include light bulbs, appliances, solar panels.

Conversion pages

These pages convert watt to or from another unit in power.

No conversion pages use this unit yet.

Other Power units

• Kilowatt (kW)
• Horsepower (hp)
• Megawatt (MW)
• Metric horsepower (PS)
• BTU per hour (BTU/h)

Sources

• NIST SP 811 NIST SP 811, App. B (power). Mechanical hp = 550 ft·lbf/s ≈ 745.70 W; metric hp (PS) = 75 kgf·m/s = 735.49875 W · Dataset reviewed Nov 11, 2026