Does geothermal heating provide 100% of my heating and air conditioning?

Groundwater, and the air temperature in caves, for that matter, is pretty stable. It ranges between 52 and 58, depending on where you are. That means it's nice and cool in the summer. Makes great a/c. And it's definitely warmer than the outside in winter if you live in a cold area. It'll keep you from freezing, but it would be pretty chilly by itself. Most people would want to supplement the heat in winter - though geothermal is definitely a good start, so it wouldn't take a lot more. (heating air from 52 to 68 doesn't take nearly the energy that heating a house to 68 when it's 20 degrees out would take!)