Meeting future inertia requirements and ensuring grid frequency stability are key challenges resulting from the energy transition. One solution is the hydraulic variable inertia flywheel, developed at Flensburg University of Applied Sciences. When designing the flywheel, sustainability aspects must be considered alongside functionality. Therefore, this article aims to identify the most sustainable material for construction. In addition to conventional manufacturing materials such as steel and aluminium, the three most common 3D printing filaments – acrylonitrile butadiene styrene (ABS), polylactic acid (PLA), and polyethylene terephthalate glycol-modified (PETG) – are considered. A stress calculation is used to determine the required wall thicknesses and material masses for various flywheel sizes. Subsequently, the energy output during a frequency decrease from 50 Hz to 47.5 Hz is calculated. To assess sustainability, the carbon-specific energy in kWh/kg CO₂ equivalent is evaluated. The carbon-specific energy increases steadily with increasing radius up to the strength limit, which is reached at a radius of 0.48 m. As steel is the most sustainable material, a construction made out of steel with largest possible radii is recommended. This article emphasises the importance of a sustainable optimisation and provides important findings for the further development of the hydraulic variable inertia flywheel.