This guide explains how to connect geothermal power and how power storage works in Satisfactory.
Starting with geothermal power, at the end of the caterium research area in the MAM you unlock geothermal power generators. They can be built on top of Geysers and connected to the power grid with no other requirements to start producing power. The power output does fluctuate between 200 and 600 MW for a pure Geyser with an average of 400 MW. It will keep rising from 200 MW up to 600 MW and then steadily fall back down. For it to be reliable you will need some form of power storage added to the grid.
There is also a building for that, which is called the power storage. They can charge and discharge at a rate of 100 MW each and store 1 hour of charge, or 100 MWh. It is one of the buildings where you can change then together using cables, there is no need to put down power poles for each of the power storage buildings.