A heat pump is a type of heater. It runs on electricity, but wait. For each unit of energy you put in you get around 5 units of heat out. Thus, it is much more energy efficient than a gas or propane heater. They are typically sized smaller than a gas heater, so you don't get rapid temperature rise with them - but once you raise the temperature they will maintain it very well. The only heater that is less costly to run is solar.