Generators are specialized facets which operate a bit differently than do valves, weirs and gates. Instead of a two dimensional interpolation table representing head difference and discharge, generators use either a three dimensional table representing head, discharge and efficiency or a two dimensional table representing discharge and efficiency.

The figure below shows the first sort of generation table: 

Here, there may be a number of pages, each representing a percentage of gate opening for the generator. In the absence of any rules the generator assumes that it can operate at any gate opening necessary to achieve the desired discharge. More interesting, however, is that the generator can use a rule to set the gate opening. In that case, it will operate at the desired percentage of opening until that value is changed.

The second sort of generator table is shown below:

In this table, each page represents a head. The generator determines the head at the beginning of the time step and then operates within the limits of the available discharge to generate power at the most efficient flow.

As with normal facets, generators are consulted during the outflow slot's begin time step to help determine the upper limit on release. They are again consulted during finish time step to determine the actual amount of water released and the amount of power generated.