When we refer to electric resistance heat as being 100% efficient, we are referring to the efficiency at converting the electricity coming into your home into heat. If we look at the source energy used to produce this current, though, the efficiency is much lower. Most coal and nuclear power plants are only 30% to 35% efficient at generating electricity. Most of the rest is lost as heat; some is lost in transmission.
Thermoelectric power plants, which account for 89% of U.S. power generation, use a fuel to heat water to produce high-pressure steam. That steam spins a turbine, generating the electricity, and the leftover heat gets dumped into the environment, such as a nearby waterway, as thermal pollution.
The overall efficiency of power plants can be improved dramatically through cogeneration or combined heat and power (CHP), in which waste heat is captured and utilized, but that is rarely done in this country.
When electricity for heating is site-generated with renewables, a very different picture emerges. With solar-, wind-, and hydro-electric systems, there are no transmission lines or steam turbines with their inherent production of waste heat. This isn’t to say that renewable power generation is 100% efficient, but the lost energy is not derived from fossil fuel combustion or nuclear fission, both of which carry environmental impacts.