One of my clients is panicing because the company he colocates his server with had a power outtage. The colocation company provides UPSes, and even has a large generator outside good for three weeks of power. But the generator only kicks in if the power to the building goes down. It does no good of power to just their office goes down, or a circuit braker trips.
I told my client not to totally rely on the colocation company for things like UPS—he should provide his own just in case. Besides, with his own UPS, he can have his server monitor the UPS and if it trips, send out a message and shut itself down cleanly (if the power goes down too low).