Geothermal power is a highly geographically variable renewable energy source. Traditionally, energy is produced from geothermal resources that have a combination of three attributes: high temperature rock, saturation of the rock mass and high permeability to allow water (the heat transfer agent) to move throughout the target geothermal field. Since geothermal power output can be maintained at a steady rate, it can be used as a renewable source of base load, unlike wind and solar.