The solution is to complement the UPS system with a back up generator that starts up if the battery autonomy is threatened, then runs indefinitely if necessary. The UPS bridges the power gap between the mains failing, and the standby generator starting up and reaching synchronisation. It also cleans all power, whether from the mains or the generator.
A number of factors apply when planning a generator installation; these relate to maintenance and fuelling, compatibility with the UPS and power system, correct generator sizing, and physical integration into the target environment. All must be taken into account to ensure a generator/UPS pair that truly guarantees uninterrupted availability, even during extended power outages.