The watt measures power. One watt equals one joule of energy per second. A typical LED bulb uses 10 watts. A microwave uses 1,000. A wind turbine can produce 5 million. The unit is named after James Watt, whose improvements to the steam engine helped launch the Industrial Revolution.

Watts are often confused with watt-hours. Power is a rate. Energy is a total. A 100-watt bulb running for 10 hours uses 1,000 watt-hours, or 1 kilowatt-hour. The bulb's power rating stays at 100 watts regardless of how long it runs. The energy consumed grows with time.

The unit scales easily. A kilowatt is 1,000 watts. A megawatt is 1 million. A gigawatt is 1 billion. A terawatt is 1 trillion. Global power demand is measured in terawatts. A single large power plant produces around 1 gigawatt. These prefixes make it possible to talk about everything from phone chargers to national grids.

Common units

• Watt: one joule per second
• Kilowatt: 1,000 watts
• Megawatt: 1 million watts
• Gigawatt: 1 billion watts
• Terawatt: 1 trillion watts