This enumerated value identifies the current external power source feeding the UPS, such as the primary utility, a separate bypass feed, a secondary utility, a generator, a flywheel, a fuel cell, or none at all if the UPS is running purely on its internal battery. It tells an admin at a glance why the UPS might be behaving differently than usual, for instance confirming a generator has picked up the load during an extended outage. For example, seeing upsInputSource change from primaryUtility(3) to generator(6) confirms the facility's backup generator has successfully taken over.