In this chapter, the complete drive system is described, which consist of an overshot waterwheel running with constant rotational speed, the gear box, and the generator. If the system is speed variable, a frequency inverter can be added. In both cases, it is important to calculate the total output power and efficiency of the system. The gear box can be chosen if the output power and the torque of the waterwheel are known. For this, of course, the individual efficiencies of the system components must be known. The total efficiency of the drive train can then be used to calculate the annual energy production of the system on the base of the duration curves of the head difference and the flow rate at the site.