Power is the the third resource and combined with Tiberium or Crystals it is used to build, hire and upgrade buildings and resources of all kinds.
Power Plants produce power and it is stored by an Accumulator.
In the early stages it is easy to over produce Power and find the storage halting as it maxes out quickly... but once you start to become proficient with your attacks you might find power drained massively as it gets used up in repairs and upgrades.