The number on the front of a UPS is a charge reading. It is not a health reading. A unit showing full can drop the load the moment it transfers. The display will have been telling you everything was fine right up until the instant it stopped being fine, and it was not lying — it was answering a different question than the one you thought you were asking. That gap between charged and working is the whole subject here. Sizing the unit is settled territory: VA against watts, runtime again...