In transmitting power, energy losses both technical and non-technical occur within the substations and should always be monitored. In this paper, energy loss assessment is carried out in detail using Statistical data collected for two months from all substations under Onitsha 330/132KV transmission substation. In all six substations, statistical data were collected; analysed then energy loss and average energy loss for the month of January and February, 2016 were calculated to be -0.03% and 0.14% respectively which was used to determine how efficient and effective Onitsha 330/132kV transmission substation was transmitting and distributing power during the period. Graphs were plotted using MATCAD to show the percentage of energy loss and effect of station service on energy loss for two months. Probable observed problems where pointed out and suggestions were given to reduce losses and to improve system efficiency.