Energy production of a power plant is significantly affected by the availability of spare parts for machinery operation and maintenance (O&M). The availability of spare parts prevents prolonged breakdown and maintenance. However, even though some quantitative methods exist to determine the number of spare parts, to avoid the events occurring, generally companies follow manufacturer recommendation that leads to depositing a large number of spare parts. Retaining excessive stocks increases inventory costs. This paper presents a novel methodology to determine the type and number of spare parts of a power plant. The methodology, which is a blending of quantitative (modified multi-criteria classification method) and qualitative (semi-Delphi approach) methods, offers a practical way that even can be exercised with paper and pencil and a simple calculator, but sound. The methodology provides a significant inventory saving. A case study of dual-fuel power plants showed that the number of spare parts recommended was less than the number currently stocked by the company. The percentage is reduced to ~ 70% with cost saving on average 6.1%. The proposed methods suggested stocking parts categorized into H (highly required) and M (recommended) types of spare parts only. © King Fahd University of Petroleum & Minerals 2024.