President Joe Biden has set ambitious goals for fighting climate change: To cut U.S. carbon emissions in half by 2030 and to have a net-zero carbon economy by 2050. The plan requires electricity generation—the easiest economic sector to green, analysts say—to be carbon-free by 2035.

Where is all that clean electricity going to come from?

A few figures from the U.S. Energy Information Administration (EIA) illustrate the challenge. In 2020 the United States generated about four trillion kilowatt-hours of electricity. Some 60 percent of that came from burning fossil fuels, mostly natural gas, in some 10,000 generators, large and small, around the country. All of that electricity will need to be replaced—and more, because demand for electricity is