A reliable power supply service is a fundamental for an economic development of one country. The problems on the quality service of power transmission and distribution systems in Ethiopia are highly affecting the investments and projects undergoing throughout the country. Addis Ababa science and Technology University is one of the huge educational institutions that suffer from this power interruption issue. This thesis has been done through identifying the existing problems on the power distribution lines supplying for the AASTU by collecting the recorded data from each substations that the lines are routed out. A reliability of the power service from two lines is evaluated in terms two metrics that show the duration and frequency of outages of the power distribution lines. The evaluation has been done with the respect of distribution system reliability indices which are SAIFI, and SAIDI. Based on the collected data of one year from the substations, the monthly average power interruption on the university is 56.06 times in frequency and 90.6 hours in duration. The main faults on the both distribution lines are short circuit and earth faults. In order to overcome the above problems AASTU is using a diesel generator as standby power source when the grid power supply is interrupted and this forces the university to cost big money for fuel. So this thesis provides a solution; a design of grid connected solar power supply that covers only the essential parts of the university which is covered with the diesel generator during the interruption of main lines. According to the data from Ethiopian meteorology agency, AASTU is located at one of the places where a sufficient solar radiation exists in Ethiopia throughout a year, so photovoltaic cells are used to design a solar energy as the main source for the essential load of the university (AASTU). The annual energy consumption of AASTU is calculated to be 18,606,240KWh/y for the selected most important essential loads only. The PV size to supply the indicated load is 8761.5KW and the capacity of the battery and inverter are also calculated as 4978125AH and 2442.6 KW respectively. The overall calculated cost of electricity for the system is calculated as 0.52USD/KWh. Based on the essential load of AASTU, the annual electric demand the PV system is selected and designed. The general over view of solar PV system starting from demand to design are explained in this thesis. The size of the PV panels, battery, inverter and cable are selected. Price for electricity in US dollars/kWh is also calculated. Key words: Power Distribution System, Voltage Drop, Reliability Indices, Photovoltaic Cells,