In a post-coal National Electricity Market, supporting solar and battery technologies can stabilize electricity prices. The optimized capacity mix includes utility-scale solar, onshore wind, and battery storage, with a focus on solar and BESS. Battery storage is modeled as an active participant, tracking behavior in real-time based on market conditions. An optimization framework is used to determine the optimal capacity mix, aiming to minimize total system cost while ensuring economic viability and reasonable prices for consumers. The analysis focuses on price formation, investment incentives, and the challenges of optimizing capacity mix in a highly renewable, post-coal system.