In recent years，ICT systems have been increasing rapidly in use in industries，businesses，residential areas，and so on. These systems play a very important role in modern society and human life. For stable operation of the ICT systems without interruption，the electric power quality and reliability(PQR) must meet certain requirements for each type of load equipment. To improve and maintain PQR and to meet the requirements，various types of backup power supplies must be installed from a standpoint of customer. It is advisable to estimate the cost benefits before introducing an expensive backup power supply. However，cost-benefit analysis
is very difficult because of unknown and uncertain parameters，such as interruption frequency and economical damage. In this paper，a general expression for cost-benefit analysis is reported. The expression uses discounted cash flow (DCF) techniques such as the net present value (NPV) and interruption frequencies obtained through reports and field surveys in Japan from the past 4 years.