Chemolithotrophic microorganisms are key primary producers in hydrothermal environments. However, the complex thermal and compositional gradients that frequently describe these settings commonly obfuscate which reactions are fueling such complex ecosystems. Nonetheless, potential sources of microbial energy can be identified by combining analytical geochemical data from hydrothermal systems and thermodynamic calculations. This approach provides a quantitative assessment of how habitats are shaped by environmental conditions such as temperature, pressure, pH and the concentrations of electron donors and acceptors. In this study, we have calculated the Gibbs energy available from 730 redox reactions in 30 terrestrial, shallow-sea, and deep-sea hydrothermal systems around the world (326 geochemical datasets) to reveal trends in how energy availability can shape hydrothermal ecology. The most energy-yielding (exergonic) reactions were predominantly the reduction of O2, NO2, NO3, and MnO2 and the oxidation of Fe2+, pyrite, CO, and CH4. In contrast, the reduction of N2, CO, and CO2 and oxidation of N2, Mn2+, and NO2, though still often exergonic, yielded significantly less energy. Also, our results show that, in terms Gibbs energies of reactions, shallow-sea hydrothermal vent systems are more like terrestrial hot springs than deep-sea hydrothermal systems. Per kilogram of water in hydrothermal fluid, energy yields from inorganic redox reactions are much higher in deep-sea hydrothermal systems than in the other systems considered here. Our results provide a comprehensive view of the distribution of energy supplies from redox reactions in high-temperature ecosystems on a global scale.