An approach often used to estimate power-system reliability worth is to determine consumers' monetary losses resulting from service interruptions, i.e. the cost of unreliability. Previously, studies have been conducted to provide estimates of customer interruption costs, and a wide range of methodologies has evolved. There is no universal agreement on the appropriateness of methodologies to particular situations nor on the interpretation of the results obtained, but some appear to be more acceptable and useful to the industry than others. A survey is presented of the techniques available for estimating customer interruption costs, the rationale of those that are currently popular is discussed, and the application of such cost data in creating a composite customer damage function is explored.>