The structure of an energy system, from production through to consumption, is fundamentally influenced by the physical properties of the energy medium, and in particular its transportability. For oil, which is highly transportable,.the system optimizes on a world scale. Electrical systems, in contrast, optimize over areas hundreds of kilometers in diameter. In future systems nuclear reactors may be interfaced with hydrogen as the energy medium. If the hydrogen is in gaseous form, the optimal system configuration will be at the level of a continent; if the hydrogen is liquid, then, as in the case of oil, the system optimizes at the world level. Generating centers will then be optimally very large: the paper gives a rapid overview of the main problems associated with such “energy islands” together with some possible solutions.