Due to high growth in electricity demand, it is important to add more dispatchable generation capacity in power systems to maintain demand-supply balance in real time. In many power systems, fossil fuel-based units such as gas or coal power plants are used to maintain high-reliability standards during peak demand hours. Although intermittent renewable energy generation facilities such as solar PV systems are used as a fuel saver, they can offset part of the load during peak hours. Capacity credit measures the contribution of a power plant to reliably meet demand. Estimating capacity credit of solar PV systems is important to ensure the adequacy and security of electricity supply. In this article, the capacity credit of solar PV power plants in the Main Interconnected System (MIS) of Oman is modelled. Oman's MIS considered in this study has 6,372 MW gas-based generation capacity, a negligible solar PV capacity, and 5,712 MW peak load. The results revealed that the capacity credit of solar PV plants is high at low solar PV penetration levels. However, it decreases drastically as solar PV capacity increases. For example, the first 500MW plant has a capacity credit of about 120 MW. When the cumulative installed solar PV capacity is 2000MW, the next 500MW plant has a capacity credit of only 26 MW.