Considerable attention is being devoted to quantitative determination of electric power system reliability worth. A basic approach is to utilise customer interruption costs obtained from postal or in person surveys. The results obtained are transformed into customer damage functions which are applicable to individual customer classes and sectors. Standard customer damage functions utilise average customer costs for selected outage durations. The paper presents a new approach which provides a three-dimensional representation of the probability distributions associated with customer outage costs. The results are utilised in a basic generating capacity adequacy assessment analysis to calculate an interrupted energy assessment rate, which provides an overall average cost per kilowatt-hour of unsupplied energy. This cost value is considerably higher than the value obtained using a conventional customer damage function.