The efficiency of an electric furnace is usually:

• A. 100 percent
• B. 10 percent
• C. 80 percent
• D. 50 percent

Answer: (A)

The main idea here is how electric heating uses energy. An electric furnace relies on resistance heating, which turns electrical energy directly into heat. Since there’s no fuel combustion or exhaust to lose energy, the electrical input is almost entirely converted into heat energy. In practice, only small losses to the surroundings or through insulation occur, but they don’t change the fact that the energy entering the system is transformed into heat with essentially 100 percent efficiency. That’s why 100 percent is the best answer. The other numbers would imply substantial energy losses before becoming heat, which doesn’t apply to electric resistance heating.