Tom,
I would say you should select the UPS based on the following:
- Capacity - Based on electrical demand of equipment you want to power, now and in the future
- Batteries - Based on how long you want equipment to run without utility power -- more batteries, longer runtime
- Topology - Online or offline, depending on your need for reliability and continuous power conditioning
- Configuration - Based on the number and types of outlets and voltage your equipment needs
- Communications - Based on need to shutdown host equipment automatically in case of power outage and/or monitor UPS status via network
- Form factor - Based on need for rackmount ability
- Cost - Your budget (check shipping costs)
- Manufacturer - Popular US brands are APC, MGE, Emerson/Liebert and Powerware
Keep in mind that without cooling, most equipment will overheat sooner or later, so let that guide your runtime goal. Here is a pretty good layman's guide to UPSes:
Information about power UPS , Uninterruptible Power Supplies and Power Protection
Hope that helps.
Ken