A modular UPS system is provided wherein UPS systems may be constructed using a number of different frame components. In one example, a core UPS frame is provided that can accept different combinations of power and battery modules, and an extension frame is provided that can be combined with the core frame to provide a larger UPS system. According to one example, both the core and extension frame types may be used in either a standalone or rack-mounted configuration. Another aspect relates to packaging of UPS components such that the UPS frame need not include serviceable items. In another aspect, a rail system is provided that allows for easier installation of heavy components into rack-mount systems.