This study investigates sustainable alternatives to diesel generators for data centre backup power, focusing on renewable diesel (HVO), Hydrogen energy storage (HES), batteries (Lithium-ion and Sodium Sulfur) and Compressed Air Energy Storage (CAES). As environmental scrutiny of data centres grows, the need for cleaner energy sources intensifies. Our research assesses various storage technologies' energy performance metrics, environmental impacts, and economic feasibility. HVO is a seamless substitute for conventional diesel, compatible with existing infrastructure and less carbon-intensive. CAES offers lower life cycle emissions and operational costs but is geographically dependent. While currently more costly, batteries could achieve better economics with increased operational hours. However, extending the backup duration increases their capital and operating costs significantly, which is less advantageous than other technologies, where only fuel costs increase with longer backup times. For existing data centres transitioning to sustainable energy, HVO is optimal; for new facilities, CAES is ideal if geography allows, with HES as a robust alternative. This analysis offers a pathway for data centres to adopt sustainable, cost-effective energy storage solutions and reduce carbon footprints through on-site renewables or green energy procurement.