- This paper presents a method for the calculation of Frequency and Duration (F&D) indices for electric power generating systems with energy limited units. Several types of energy limited units are discussed. For an energy limited unit, a critical load level at which this unit is put into operation is found by peak-shaving method and then the cumulative probability and frequency are computed by conditional convolution. The method is illustrated by numerical examples. Copyright 1987 by The Institute of Electrical and Electronics Engineers, Inc.