In the summer of 1992, Korean electricity system suffered from insufficient electricity generation capacity. Mainly, it was due to rapid increase in peak demand for electricity. However, it is also due to incorrect assessment of optimal reliability level which is one of important input factors for the Long-term Electric System Expansion Plan. Fortunately, the problem could be overcome through two lines of efforts, those are, individual's voluntary electricity conservation and mandatory prohibition of cooling system in public buildings. This study focuses on the estimation of correct shortage cost for electricity. The estimation is performed by using national input-output table. Besides, Literature in this field are systematically surveyed. Also, efforts are made to estimate marginal shortage cost as the above result is based on average concept.