The goal of bulk power system planning is to ensure that sufficient energy resources and electric transmission infrastructure are installed to serve demand. System planners use forecasts of future demand along with existing and planned resources to determine, on a probabilistic basis, if those resources will be sufficient in meeting reliability targets. Several identified trends within North American Electric Reliability Corporation (NERC) Assessment Areas for the North American Bulk Power System (BPS) are showing tightening Reserve Margins year-to-year from previous assessments. These trends, combined with the ongoing changes to future generation portfolios and load conditions, create uncertainties and potential reliability risks to the BPS. This paper addresses the uncertain performance of Variable Energy Resources (VER), such as wind and solar, and provides stochastic models that have been incorporated into the Electric Reliability Council of Texas (ERCOT) interconnected system. Wind and solar capacity factors, using a time series method, are presented as well as a Monte Carlo simulation that has been carried out to show the impact of uncertain parameters affecting Reserve Margin calculations. Lastly, a probabilistic sensitivity analysis has been performed; these results are presented by parameter significance.