 This is because gas heaters, especially when unflued, are not suitable for bedroom use - gas heaters cannot be used in confined spaces due to the reduced oxygen, and the emissions produced.This leaves electrically powered heaters, such as oil heaters, fan heaters and heat pumps, as the only alternative.Several efficiency metrics can be measured in regard to heaters, such as the efficiency of heating a room with a given amount of power, and the efficiency of the electrical generator which powers the heater and power loss from transporting the electricity over power lines.