In this master thesis outage statistics covering tree years from two distribution grids are analyzed with the purpose to deduce which factors yield outages. The statistics are extensive therefore allowing in-depth analysis of both customer and system related factors. From this starting point and as a model of explanation energy not supplied is studied as the consequence and the measurement of the inconvenience caused by outages. Different methods for calculating energy not supplied are developed and studied and put in comparison to the method used in Network Performance Assessment Model, which is the Swedish frame work for regulating electricity distribution. Analysis of the outage statistics show a relation between the type category of customer and outages that can be deduced to the variance and the density associated with that customer category. Prioritization of different customer categories is elevated also in this aspect. The variation in outage frequency between the different categories and parts of the grid is to be proven significant in relation to the energy not supplied, which has impact of the assessment of the Customer Reliability Value in the Network Performance Assessment Model. The calculation of the amount of redundancy performed in the Network Performance Assessment Model is also influenced by the method of calculation for energy not supplied. Finally suggestions of alternatives for diversifying this calculation are made.