A fuel cell power pack comprises a base module, a ballast module, a fuel supply module, a generator module and an enclosure consisting in part of panels, that is shaped to fit within the battery bay of an electric vehicle. The base and ballast modules are configured to provide ballast for the electric vehicle, to hold the fuel supply module, and to form part of the power pack enclosure. The fuel supply module comprises a fuel storage cylinder and a length-minimized fuel supply assembly to provide a maximized fuel supply to the generator module. The generator module comprises a fuel cell stack and balance of plant components operable to generate electricity. An explosion dissipation structure is provided on at least one enclosure panel.