The Sun is overall source of energy for everything on the plant.  Energy, or the property of a system that enables it to do work, is literally everywhere.  There are two forms of energy, potential and kinetic.  Potential energy is the stored energy that a body possesses because of its position.  Kinetic energy is the energy of the object's motion. 

Energy cannot be created or destroyed (conservation of energy), however, it can change form, such as being converted from water energy to electric energy (energy conversion).  The momentum, or the product of the mass and the velocity, of the river as it comes through the dam, is used to create electricity.  The momentum of the river changes (impulse) and the machines inside of the dam convert the water into the electricity.