A power assembly for dispensing product from a container includes a pump mechanism and an energy storage mechanism. The pump mechanism includes a rotatable actuating collar connected through an escapement mechanism with a drive screw with an externally threaded shaft engaged with internal threads in a piston. Rotation of the collar reciprocates the piston to store energy in the energy storage mechanism and to draw product into a pump chamber from which the product is dispensed. An actuator is connected with a stem valve to open it when the actuator is depressed, enabling the energy storage mechanism to push the piston back to its at-rest position to force product from the pump chamber and ready it for another dispensing cycle. Disengagement of the escapement mechanism when the actuator is depressed prevents rotation of the actuating collar. Different energy storage mechanisms can be interchangeably attached to the pump mechanism.