Power is the rate at which energy is used or transferred. A 100-watt bulb uses 100 joules every second. A 1,000-watt microwave uses ten times that. Power tells you how fast, not how much. Energy is the total. Power is the speed.

The unit is the watt, named after James Watt, who improved the steam engine. One watt equals one joule per second. A kilowatt is 1,000 watts. A megawatt is one million. A gigawatt is one billion. These units describe everything from phone chargers to power plants.

Power matters for grid planning. A utility must meet both total energy demand over a year and peak power demand at any instant. A plant that runs at 500 megawatts for an hour produces 500 megawatt-hours of energy. The distinction between power and energy trips up many people, including journalists and politicians.

Common units

• Watt: one joule per second
• Kilowatt: 1,000 watts
• Megawatt: 1 million watts
• Gigawatt: 1 billion watts
• Horsepower: about 746 watts