Despite its name, the Energy Department's single largest responsibility is managing the U.S. nuclear weapons stockpile through the National Nuclear Security Administration, which designs, maintains, and secures America's nuclear arsenal without conducting live nuclear test explosions, which have been banned by policy since 1992.

The department also manages the environmental cleanup of former nuclear weapons production sites, oversees the Strategic Petroleum Reserve, the world's largest emergency crude oil stockpile, and funds a wide range of energy research and development, from advanced nuclear reactor designs to renewable energy technology.

The department runs seventeen national laboratories, including Los Alamos, Lawrence Livermore, and Oak Ridge, which conduct fundamental scientific research well beyond energy alone, spanning particle physics, supercomputing, and materials science, making the department one of the largest funders of basic scientific research in the federal government.