A geothermal generator is a structure that can only be built on a geyser and converts the thermal energy into electrical power. The geothermal generator is available once it is researched (with cost of 1800 research points). It produces just over twice as much power as a solar panel, at just over three times the cost. It runs day or night, unlike a solar generator or wind turbine.
In cold biomes it can be useful to build your initial base surrounding a geothermal vent as a source of heat, and eventually power generation.
Geothermal generators have a very low beauty score, so if placed indoors, should have their own room. While you can use vents to pipe the generator's heat into another room, a cheaper alternative is an open door. The open door will conduct heat, and colonists in adjacent rooms will not notice the generator through the door, even if it's in line of sight.