The paper proposes new methodology for development of customized damage functions for evaluation of financial losses incurred by industrial users due to voltage sags and short interruptions. The existing customer damage functions (CDF), developed based on past surveys, are suitably scaled and transformed into comparable platform using financial conversions. The modified CDFs are then compared with the target user performance and combined into a new damage function unique to the target user. The paper provides detailed step-by-step procedure for implementation of the methodology in order to facilitate its practical use.