the big point about having a ups is not to provide you indefinite amounts of power, but upon the loss of power, you'd have time to save any files you were working on & shut down your machine.

Here at Stony Brook, the computer center has a room specifically for the their ups, but if they don't shut down systems, the whole east coast of the Internet would go down after 15 minutes. The ups allows them to shut down stuff, transfer to other servers, & put their servers into sleep modes.