Savonius turbines are practical for rivers with low speeds due to their simple design and good self-starting capabilities. Despite these advantages, real implementation is still limited due to their low power efficiency. However, the experimental studies are scarce as they mainly focus on power performance, neglecting the impact of materials exposed to the wind or water medium. Therefore, the current study aims to address this gap by investigating the influence of materials on the power performance of Savonius turbines in wind and water, particularly the effect of their mass moment of inertia (I). Aluminum (ALU) and Polylactic Acid (PLA) were selected as the materials for the turbines as they are the typical materials frequently used to fabricate the turbines. They were tested experimentally in a wind tunnel and a water flume with Reynolds numbers ranging from 52, 000 to 83, 600. The findings discovered that although the turbine materials have the exact mass moments of inertia, they perform differently in water and wind. The turbines exhibit higher power coefficient with increasing mass moment of inertia in the wind at low Reynolds numbers (Re < 70,000), which is most likely due to reduced bearing friction that is more pronounced in the low Reynolds number region. The power coefficient was more likely to be affected in the wind than in water due to the effect of water buoyancy force, which reduces the influence of the increased mass moment of inertia. The results also revealed that the ALU and PLA with the same overall mass moment of inertia had maximum power coefficients of 11% and 18% in wind and water, respectively. There is no significant difference in terms of self-starting speed due to the variation in the moment of inertia. The Reynolds number and the turbine starting angle appeared to have a greater effect on self-starting capabilities than the moment of inertia. The findings of this study help to establish the types of materials that are optimal for enhancing the turbine performance in wind or water applications and could potentially save a lot of costs in generating power.