Hi Soulwindows!
Mike's answer is a good one. I'd just like to add a little more in case it helps your understanding. The back up system kicks in only when the heat pump needs to make up a big temperature difference (between the current temperature, and the temperature you want it to be). This happens most often when temperatures outide get really low (like 20 degrees or lower) or you turn the thermostat way up. The heat pump "knows" it has to produce a lot of heat to get to your desired temp, so it engages the back up system (think of it like a big toaster). It is VERY inefficient when the back up is running, but very efficient under normal circumstances.
It might help to know that the back up system will automatically kick into action whn you turn the thermostat up more than 3 degrees above the existing temp in the home. For this reason, it is recommended to keep the thermostat a relatively steady, but comfortable setting - as opposed to turning it way down, then spiking it up when you get home from work, for example.
Hope that helps. Here are 3 good sites (mostly about energy saving tips) for info that should provide more complete and (hopefully) useful info:
http://home.howstuffworks.com/home-improvement/heating-and-c
http://www1.eere.energy.gov/consumer/tips/heat_pumps.html
http://www.doityourself.com/stry/heatpumpenergyuse
Hope that helps!
- Tue Oct 6 2009, 17:35