How does an elevator work?
When you step into an elevator and close the door, you had passed through two doors and are now standing in a box (or the elevator car) inside a vertical passageway (called the lift shaft). One door is in the walls of the floor that you got off, and the other door is part of the car itself.
Inside the shaft are hoisting cables attached to the top of the car. The cables run over a sheave (pulley) connected to an electric motor at the top of the shaft. The other end of the cables is connected to a heavy steel weight called a counterweight. When the car goes up, the counterweight goes down; when the car goes down, the counterweight goes up.
How the counterweight reduces to a minimum the power needed to operate the elevator ...
Weight of counterweight = Weight of the car + (about) ½ of its maximum passenger load
So when the elevator operates, it needs power only to lift the weight of the extra passengers in the car; the rest of the weight is balanced by the counterweight.
Flash movie of an elevator
"George Files" by Parenting the Next Generation