The power grid is the backbone of any factory, providing electrical power to all facilities. Any Generators attached to a power grid work together to power everything connected to that grid. If a grid's power demand is greater than the power generation provided to it, facilities attached to the grid will run slower or even stop altogether.
Generators
Wind Turbine
Wind Turbines are a Tier 1 power source, and accessible from the very start of the game. Turbines are very cheap to build, but do not generate much power. They also require quite a bit of clearance around them, preventing any form of high-density power generation. Finally, the power produced by the turbines is also dependent on the planet, as wind speed varies planet to planet.
Thermal Power Plant
The Thermal Power Plant is a Tier 2 power source you gain access to early on. These allow for higher-density power generation but require a constant flow of fuel. Running out of fuel will cause the factory to grind to a halt, including any mining machines and sorters that would otherwise be providing fuel. Over-reliance on thermal generators can cause cascading power failure as a result; if enough power plants run out of fuel such that facilities begin to slow down, the fuel supply to the remaining thermal generators will slow down until those too run out of fuel.
How long a piece of fuel burns for is based on its MJ value. Thermal generators suffer a 20% loss due to inefficiency. To calculate how long one fuel unit will last, multiply its MJ rating by 80% (or 0.8), then divide it by the 2.16MW that the generator produces per second. The resulting number will be how long one unit of that fuel will last.
Example 1: Coal has a energy density of 2.7MJ. Take away the efficiency loss (2.7 * 0.8) and you are left with 2.16MJ. Divide that by the thermal generator's max production (2.16MJ / 2.16MW) and you get that one piece of coal will burn for 1s at full use.
Example 2: Hydrogen Fuel Rods have a energy density of 40MJ. Take away the efficiency loss (40MJ * 0.8) and you are left with 32MJ. Divide that by the thermal generator's max production (32MJ / 2.16MW) and you get that one Hydrogen Fuel Rod will burn for 14.8 seconds at full use.
Solar Panels
Solar Panels are another Tier 2 power source you gain access to early on. Much like wind power, each panel is weak; however, they do not have a spacing requirement, so players can place many more solar panels together much more closely than turbines. Solar panels' big drawback is they only function during the day, requiring accumulators to store excess power for use during the night. Notably, if a planet has a relatively low obliquity and orbital inclination, solar panels placed on its poles will virtually always generate power. The power produced by the panels is also dependent on the planet, as solar intensity varies planet to planet.
Mini Fusion Power Plant
The Mini Fusion Power Plant is a Tier 3 energy source that is gained late into the tech tree. These consume Deuteron Fuel Rods to produce power, similar to Thermal Power Plants. As such, they have the similar caveat that If you run out of fuel, the production lines producing Deuteron Fuel Rods will most likely stop, causing a cascade of power failures.
Deuteron Fuel Rods last for approximately 40 seconds in the fusion plant, producing 15 MJ of energy per second.
Artificial Star
The Artificial Star is a Tier 14 energy source and the last type of generator the player unlocks. It consumes Antimatter Fuel Rods and provides 4.8 times more energy than the Mini Fusion Power Plant.
Transmission Towers
Tesla Tower
The Tesla Tower is the main power pole you use early game. It allows you to connect generators to your buildings and spread the range of the power grid. It has a decent supply area, but suffers when trying to run long distance.
Wireless Power Tower
The Wireless Power Tower is the long-range power pole you use in the early game. Although its supply area is considerably smaller than the Tesla Tower, it allows for longer-reaching connections spanning greater distances. It can also recharge the Core in the Icarus Mech if you stand next to the pole.
Satellite Substation
The Satellite Substation is the final transmission tower used in the endgame, boasting both long range and wide coverage. This transmission tower is rather expensive, but provides massive range and distant connections to other transmission towers. The Satellite Substation also takes up more space than Tesla Towers and Wireless Power Towers, so fitting it into compact factories may prove challenging.