In 2020, the U.S. produced 4,009 terawatt-hours of power (also written as 4,009 billion kWh, since one billion kWh is one TWh).

■ About 60 percent of that total came from fossil fuels like natural gas (40.3 percent), coal (19.3 percent), and petroleum (0.4 percent), all of which have CO2 emissions as an undesirable side effect.

■ Nuclear energy accounted for just under 20 percent. It is debated whether nuclear power falls under the category of “sustainable,” but at least it doesn’t emit any greenhouse gases.

■ So-called renewables make up the remaining 20 percent, the main ones being wind (8.4 percent), hydro (7.3 percent) and solar (2.3 percent). Smaller renewable energy sources are biomass (1.4 percent) and geothermal (0.4 percent).