This paper describes a probabilistic approach to computing information on off-peak energy potential of an electric power system useful for studies related to economic assessment and duty cycle prediction of energy storage devices. Effect of minimum generation limits of generators on system operation and associated economic penalties can also be evaluated by this method. The methodology is similar to generating capacity reliability evaluation. Computation of expected off-peak energy and expected capacity margin durations is illustrated by a system example and sensitivity of these quantities to generating unit size is evaluated. Suggestions are made how the existing computer programs using frequency and duration method of generating capacity reliability evaluation and probabilistic production cost evaluation method can be used to determine off-peak energy information.