There are two kinds of geothermal or underground heat. One has its source within the Earth; the other derives from the sun. The Geo Power System utilizes the underground heat from the sun. Energy absorbed by the Earth radiates slowly downward through the ground, creating a temperature difference between the underground environment and the outside air. This temperature difference is why well water or the air in a cave feels warm in the winter and cool in the summer.

The graph shows temperatures for a city in Western Japan. At six feet (1.8m) below the surface the underground temperature is affected by the outside air temperature, whereas at 30 feet (9 m) below the surface the underground temperature is hardly influenced by the outside air temperature at all and is nearly constant all year round.

It takes the energy from the sun about half a year to penetrate down to a depth of 16 feet (5 m) . This means that the underground temperature at 16 feet (5 m) is slightly lower than the average during the summer and slightly higher than average during the winter.

The Geo Power System makes use of the temperature difference between the outside air and the underground temperature at a depth of 16 feet (5 m) . This difference can reach a peak of 20 degrees Celsius (36 degrees F).