In 2025, the Duri Kosambi and Pulogadung Transmission Implementing Units, operates 157 power transformers and 905 current transformers across 54 substations. Between 2017 and 2024, there were 26 permanent failures of power transformers and 8 failures of current transformers requiring replacement. Spare provision is a crucial element in an integrated inventory management strategy aligned with strategic and asset planning to ensure efficient material availability. This study aims to determine the optimal quantity of Joint Inventory stock for 2025 by considering criticality, availability, and usage rate. The methods used include time series analysis, reliability-based availability approach, and cost minimization. Based on calculations of failure rate, lead time, and emergency costs, the optimal result is 4 units of power transformers (λ = 0.0264 and β = 0.3541) with a minimum estimated cost of IDR 21,345,844,706 and 94.08% reliability, and 2 units of current transformers with a minimum cost of IDR 80,463,294 and 96.17% reliability. These results are expected to enhance the efficiency and reliability of spare management in both units.