the company i work for has a decent sized data centre with a several petabyte SAN and more clusters than i care to remember
Each of the physical boxs has a UPS, and all the comms equipment for the Vm clusters etc.. is attached to UPS as well.
In the event of a powercut, the UPS kick in, and the backup generator outside is used instead. Its diesel driven and is about the size of a 40 foot lorry without the wheels. This then kicks in and can supply power for as long as we've got diesel for. Think we have a couple of days worth in reserve.
Just in case the worst happens, we have a failover site as well at another location.
All the SAN's are mirrored and if the worst should happen such as massive fire / flooding we can failover and just switch to the alternate location which is the other side of leeds.
the NHS contracts we hold have nasty penalities for service outages so naturally the continuity plan is geared up to ensure that there can never be any more than a few minutes downtime.
Most of the servers are VMware clusters so if one physical machine fails the image for one sever can be vmotioned elsewhere without effecting performance too much. We always have a couple of hosts spare that the image can be moved to if required without notice.
if a UPS fails we have on call engineers with pagers and UPS sat in stock in readyness. if a UPS does fail engineer will be paged and can usually get a replacement UPS fitted within an hour if needed, but they are self tested every week so this rarely happens.
obviously every business is different, some business couldnt afford an alternate location with an exact duplicate of their datacentres and a link to get all the data synced up. As already said, for some a UPS is going to enough.