They use an infrared lamp pointed at copper plates. They draw either 750 or 1500 watts of power, so the electricity bill will be going up. Electric heat is usually nowhere near as cheap as natural gas.

This guy tested a couple.

1500 watts = 5118 BTU/HR, by the way, so considering that a small gas furnace is around 70-80,000 BTU/HR, you may need a few.