When getting a UPS for critical computer systems one has to determine the goal of the UPS. The more power they are designed to handle, and for longer time periods, the more expensive (and large) they are. If the goal is to merely cover short-term power outages for a computer system, then a very small UPS is fine. It is common to not even connect your power hungry computer monitor to the UPS in these instances. However, if the goal is to provide a way to properly shut down a computer system after a power loss it is important that you consider the power needs of the monitor (which can be substantial). If you don't have a functioning monitor it is going to be impossible to properly shut down the system.

Some people have the idea that they will not connect the monitor to the UPS during normal operation and then, if there is a blackout, they can connect the monitor to the UPS to get it power to shut down the system. This is backwards logic that often fails because the power required to power up a monitor is greater than what it needs in normal operation. This technique can serve to either blow a circuit breaker on the UPS or put such a high drain on it (after having already supplied power for a few moments) that the battery only lasts a few more seconds.