Power Back-up Solution for a Computer Lab consisting 15-20 PCs ?

Clarifications

How long do they need to be up and running?

What needs to be up during the blackout? CPU's? Monitors (LCD or CRT)? Switches? Routers? Other custom equipment?

You could have one big UPS installed and hard wired into the building's power supply. Or you could purchase 15-20 little units. Depends on how much your budget is... And whether you would want to expand the unit to protect other equipment.

Where I worked we had a UPS that powered the entire data center including all tech workstations and other mission critical workstations (such as finance, and call center). The UPS was also hooked up to a generator so that we could run indefinitely (well as long as the city utilities supplied natural gas)

One safe option is to get a ups per pc and have the outlets the ups's are connected in line with a generator for long term outages, the small ups's should be sized to carry the pc for 10 min (or long engh so the generator kicks in)