A power system responsible to supply reasonable level of reliable, qualitative electricity supply to its consumers in an acceptable price. Reasonable level is defined as one balancing the need for power supply and the cost involved. This includes increasing adequacy and security of the system. If the frequency and duration of outages can be reduced, the benefit can be given to consumers. Cost of Unserved Energy is considered as a basic parameter of economic evaluation in the process of increasing the power system reliability. Initial research was conducted before two decades ago and now it is needed to conduct a study of up-to-date on the scope. Thus, research to assess the cost of Unserved Energy for Sri Lanka was conducted and the outcome of the research is presented in this research paper. Consumer survey is chosen as the methodology of the research and it is conducted into three separate electricity consumer categories of domestic, industrial and Commercial. Each category is sub-grouped according to different classifications and separate results have shown for momentary planned and unplanned and non-momentary planned and unplanned interruptions. Finally, the results of each category are averaged into two final values one each for each classification considered.