The energy from all those small red signs adds up:

Each one uses relatively little electricity, but they are on all the time. And we have a lot of them in our schools, factories, and office buildings. The U.S. Environmental Protection Agency estimates that there are more than 100 million exit signs in use today in the U.S., consuming 30–35 billion kilowatt-hours (kWh) of electricity annually. That’s the output of five or six 1,000 MW power plants, and it costs us $2-3 billion per year. Individual buildings may have thousands of exit signs in operation.

Maggie Koerth-Baker zooms out:

In the United States, we use more energy powering our buildings—from the lights, to the heating, to the stuff we plug into the walls—than we use to do anything else. Because of that (and because of the fact that electricity is mostly made by burning coal or natural gas) buildings produce more greenhouse gas emissions than cars.