"Provides a great amount of energy generating steam by burning combustible materials."
Power Plants are energy production buildings researched in the Stone Workshop, allowing the player to produce large amounts of energy fairly quickly at the cost of building materials (-10 wood income). While they are larger and more expensive than Mills, they are more space efficient, workers efficient and do not have a 4-tile spacing requirement.
Power plants consume wood. It is important that the player keeps adding extra saw mills, lest they will get constrained by wood income.
Mills are around 30% more efficient in terms of maintenance cost, but need nearly 50% more workers and more than twice more space. Placing mills requires more attention, which also makes power plants a better choice for no pause mode.
Power take almost 24 hours to build. That's 3 ticks of resource income. They should be build in advance to avoid going over resource cap.
Comparison of efficiency of different energy sources