Okay, the Dell R510 requires about 270W per server which would equal 540W for two. Multiply
this by 1.414 to get peak power drain for the UPS and then round up to the next UPS size, so
either 800W or 1kW.
In data centers, power is distributed through UPS so that when power fails for a short amount
of time, the systems keep running until the diesel generators are up and running.
There are extended run UPS systems available, but they don't come cheap. A ballpark figure
for the UPS would be about 10 times what you'd pay for the server.
Easiest way to get you started is using an existing tool
The APC website is quite handyhttp://www.apc.com/tools/ups_selector/index.cfm?
you can also check the peak and mean power consumption on the manufacturers website
p.s 2.5 hours is a hell of a long runtime for a rack mount UPS