Nowadays electricity market is changing: electrical power producing companies are being separated from transmission and distribution companies. Electrical energy producing companies are participating in electricity market, while transmission and distribution companies are regulated by some government institutions. In such situation companies under regulation can try to diminish their costs by rendering bad quality services. In such situation it is very important to define some criteria that will insure good quality of power supply. Unfortunately in legislation of Latvia there are no any parameters that would regulate reliability of power supply. In present legislation there is mentioned only one parameter that partly affects reliability – in legislation there is mentioned that any power supply interruption should be eliminated in 24 hours. Nowadays to quantify the reliability of a distribution network different indices are used. Most commonly are used – SAIFI, SAIDI, CAIDI and EENS. Disadvantage of the most existing indices is that they quantify system averages, but they don’t express feelings of customers that go through power supply interruptions and because of that we can’t express how a customer experiences the reliability of supply. In the paper there is described Customer Dissatisfaction Index. Usage of Customer Dissatisfaction Index (CDI) can help us to understand how a customer experiences the reliability of power supply. . After the introduction of the index, CDI value reduction costs are calculated. At the last part of the paper some possible ways of application of CDI and the results of calculation of CDI value reduction costs are discussed.