What’s a watt?

Watts are units of power. Power used over time is energy (which has units of “watt hours”). What’s the difference? Power is the rating for a device you use in your home (or a power plant). Energy is how much that device uses over time. So if you have a lightbulb that is rated at 10W and it’s on for two hours, that’s 20Wh of energy.

When talking about the power system these watt numbers get pretty big, pretty fast. Quick reminder about SI prefixes: kilo (as in kW) means one thousand, mega (as in MW) means one million, giga (GW) means one billion, and tera (TW) means one trillion. As a rough rule of thumb - your electricity bill gets measured in kW hours, a single generator has a power rating in MW, and all the electricity the US uses in a year in TW hours. More specifically, here are some things you’re familiar with and how many watts they are:



laptop, while in use

10 W

indoor LED light bulb

10 W

indoor incandescent light bulb

60 W

one stove top burner or an air conditioner


one LED, running all year

87.6 kWh

a full charge of an electric car battery

40 to 100 kWh

average annual US residential electricity use per person

4500 kWh

modern solar farm

4 MW

modern wind turbine

2 MW

modern US wind farm

90 MW

coal power plant

700 MW

nuclear power plant

1.5 GW

US renewable capacity

127 GW

US coal capacity

255 GW

total US power plant capacity

1.1 TW

US renewable energy generation in a year

372 TWh

US total annual electricity use

3860 TWh

Sources for these numbers and more details can be found here.

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