Geothermal energy refers to the temperature of the subsoil. The further you go down to the center of the Earth, the higher the temperature. At 100 meters deep, it is – all year – at about 12 ° C. At 300 meters, it reaches about 20 ° C. Around 400 meters, it is enough to directly heat a building, without a heat pump. And around 5000 meters, it allows to produce steam under pressure to generate electricity with a turbine. These are orders of magnitude: the temperature reached, and the quantity of heat that can be extracted, depend on the geological nature of the ground and the presence of water.