Geothermal energy uses the internal heat of our planet core, and make it possible to use this source of energy for heating or for electricity production. Restag proposes low enthalpy geothermal systems for heating and cooling homes, which uses the ground as a reservoir of heat during the cold months, with heat transferred to the surface, while in the summer the excess of heat is accumulated in the soil , operation made possible by electrical heat pumps. The heat pump uses the temperature difference between the ground and the air; more high the difference is, better the performance. The heat pump absorbs heat from the ground and makes it available for human uses; to cool, absorbs the heat from the house and transfers it to the ground.

Alternation of summer / winter cycle operation permits to have no significant change in the thermodynamical condition of ground where probes are located. Geothermal probes are U-pipes made of materials with high thermal transmittance having inside a liquid that absorbs the heat and carries it to the surface or underground. In total (since heat pumps are powered by electricity) total heating costs are less than half compared to a conventional plant run with natural gas. Restag can however develop PV energy storage systems in conjunction with the geothermal unit, to economize on the energy consumption of the heat pumps