The system will use some power. An P4 Prescott 3GHz with 2x160GB SATA disks, 1GB RAM, will shut down a 500W UPS when a 17" CRT Samsung monitor starts. While its continuus used power (for the monitor) is like 100W, it can request shocks of current of several amperes.
Also, a bigger UPS will get you more running time on batteries