Rejected energy means energy that is lost as waste heat without performing any work first. For example, a coal fired power plant may generate 3 megajoules of thermal energy from coal combustion but only deliver 1 megajoule of it as electricity. The other 2 megajoules are lost as useless waste heat.

The 1 megajoule of useful electricity is also ultimately dissipated as low grade heat, but it can do work first (like generating light, or pumping water uphill).