@thebowman: Depends. I have dipswitches on the unit that I can set which select if there is a 2-stage or 3 stage heat. 2 stage is the two speeds on the compressor. With 3 stage heat, you can set it up so that in addition to the heat pump compressor, the resistive electric strips run. The thermostats have to match (i.e. know or have their settings set for 3 stage heat). My main 5 ton unit uses restive electric for a quick warm up in the morning. My 3 ton for my work area does not since it has very few windows in it. If the power bill got to me, I could nuke the 3rd stage for the main unit and save about $20 per month during peak heating season. Typically, thermostats use 1, 2, and 3 degree difference between the call temperature (what you set it for) and the room temp to select how much heat to throw at the problem. Note also that when I have the dip switches in 2 stage mode, the system is smart enough to kick in the resistive electric if there is a compressor problem or a water temperature problem.
@roamer: The geothermal units uses about the same amount of power. For a given tonage, the compressors are the same. The water pumps that move the water in the ground loop about trade with the fan in the outside unit on a traditional heat pump. The savings are shorter run times of the unit when compared to an outside unit. Put another way, an air exchange heat pump in the San Francisco bay unit would fare about the same as a geothermal. A geothermal in New England would woop an air exchange. Most geothermal systems also have 2 speed compressors in them. I think the systems are overpriced due to low volume, and so the two speed compressor is an attempt to justify that. Biggest win of two speed compressor is when your cooling. At low speed you use less power but run longer which means you can take more water out of the air (lower humidity).
Another way to say this: I can get 98 degree air out of a register when it's 10 degrees out. That doesn't happen with an air exchange system.
Pete