This paper presents a proposal of a methodology for assessing costs due to electric service interruptions. A review of some works developed around the world related to the continuity of the electric power supply service and an examination of Colombian regulatory context on Power Quality are presented. The Electric Service Quality of industrial users in Bucaramanga and its suburbs was analyzed through a survey which asked users about their perception concerning to the power quality service and the costs provoked by power service interruptions. The results of this survey encouraged us for developing a methodology that could help the users of the service in the task of assessing costs caused by the inappropriate Power Supply Service and that could also be a fact to make them starting to think in a different way about service quality. The proposed methodology includes seventeen components which enable costs assessing in a manufacturing company incorporating the administrative area as well as the production area. Some variables analyzed are Cost of Equipment damage, Cost of damage to processing products, Cost to makeup production, Costs of inactive working labor and Cost of value of lost production. Besides, the results of applying this methodology in two manufacturing companies are shown and the variables listed above were the most affected when a power service interruption happened. This work also includes the changes which were made to the initial proposed methodology. Through this, evidences of the added value that application generates to any research work are given.