a light, the odds are that the energy comes from burning coal. Close to 60 percent of all U.S. electricity comes from coal, so power stations must be supplied 365 days a year with this most abundant fossil fuel. FRED PALMER: People don't think a lot about coal, and they are not required to think about coal. But Coal in the United States is the major driver of our economy, in that it supplies 56 percent of electricity. One 1,500-megawatt coal unit supplies electricity for a million and a half people. NARRATOR: Coal generation is supplemented with other carbon-emitting fossil fuels like oil and natural gas. Some sources of electricity produce no greenhouse gases- hydroelectric power, solar energy, the power of the wind, and the largest carbon-free source of energy, nuclear power, which currently supplies over 20 percent of U.S. electricity. All these sources flow into electricity grids, like this one managed by Northern States Power in Minneapolis. What few people realize is how much energy is being consumed 24 hours a day. Even at 3:00 in the morning, 55 percent of the load is operating. AUDREY ZIBELMAN, President, NSP Energy Marketing: Nothing totally shuts down, and even at 3:00 o'clock, you have the street lights on. You have factories that are working on a 24-hour period. And what we'll do at the night time is run our cheapest units, our coal and our nuclear units. And those are the units we call our baseload units that we use all