Since you used the plural "computers", so you're likely to be networked, I offer this bit of advice from personal experience: make sure the hub/switch/routers, monitors and any external hard drives are on the UPS, too.
And since you're talking about load shedding on a budget, don't bother with the laser printers, etc. You want a system that maintains data integrity, not productivity.
The problem with many UPSes is that they can send a "power gone" signal to only one device. You should consider a hardware doohicky that can tell all your computers that power has gone, and that so many minutes remain. That way, you can have computers start shutting themselves down as the UPS runs down, or when the generator is down to the last 2 liters of fuel.