A high penetration of wind and solar generation within the generation mix will result in an increased supply risk due to the random variations inherent in renewable generation. This document proposes a new methodology for calculating system marginal costs as part of generation fleet expansion planning that accounts for the impact of these random variations. This new methodology incorporates an additional cost into the marginal cost calculation, reflecting the risk of supply deficits during periods of scarcity. This allows for the inclusion in the planning process of generation units that contribute to the electricity system firm capacity, thereby enhancing the security of supply to meet demand. As an example, this document analyzes the economic feasibility of a generation project that utilizes hydrogen (H₂) as fuel. The project combines H₂ production for industrial use, powered by renewable generation (the GH₂ project), with the provision of firm capacity supplied by a gas turbine (TG) generator fueled by H₂ (the hybrid project). The economic feasibility of this hybrid project stems from two key factors: i) intra-annual energy arbitrage—purchasing energy from the electricity market during periods of the year characterized by low marginal costs, and selling energy during periods of scarcity with high marginal costs; and ii) maximizing the electrolyzer's capacity factor by purchasing energy in the market during hours when the electrolyzer possesses residual capacity (i.e., capacity not currently utilized by the GH₂ project). The project incorporates H₂ storage within salt domes.