APC Uninterruptible Power Supply?

The difference would only be based on the "draw" and your right the 650 SHOULD supply power longer than the 550. Time is irrelevant until you know exactly how much 'draw" each will have for how long. Obviously either on a 20W draw would last longer than either on a 300W draw. It would be safe to say the 650 would give power longer than the 550...IF...the draw was exactly the same. What they tested the two at would be the reason for the difference in time