Do you remember that energy can't be created or destroyed ? It always has to come from somewhere or go somewhere.
IF there's no friction or air resistance to steal energy from the pendulum, and nobody bumps up against it while it's swinging, then it always has the same amount of energy. The energy is all potential at the top of the swing, all kinetic energy at the bottom of the swing, and half-and-half when it's halfway between the top and bottom of the swing.
If it has 892 Joules of potential at the top, it'll have 892 Joules of kinetic energy at the bottom. Or whatever the number happens to be.
