Due to the threat of climate change and its negative impacts as well as the fact of non-sustainability of fossil fuels, there has been a significant increase in interest on renewable energy systems. Solar energy is an ideal source of renewable energy as the total solar energy reaching to the Earth's surface in a year is more than 10,000 times of the world's proven reserves of energy. However, higher capital cost is one of the major drawbacks of the solar systems. Lack of reliability can have a negative effect on the manufacturer's reputation and in some cases the perception of the technology. Lower reliability also increases the cost of energy produced. So reliability power system is one of the important aspects for solar energy systems. The greatest challenge for solar system reliability estimation is the uncertain system input. This paper proposes model for estimating reliability and availability of On-Grid Solar PV System - a solar (PV) power plant integrated into the distribution network. The developed model is then illustrated with a numerical example.