A 3000VA UPS could support about five servers. At about AU$6000 per server that's AU$30,000. Now how much value do you place on the data stored on those machines? How many person hours will it cost to replace or repair hardware, load backup data, and then redo the work that was lost between the last backup and the power crisis that has stopped your business in its tracks. (You do have recent backups of all your data on removeable media don't you? And that media is in a safe place, yes?) Even if no physical damage is done, even if no data is lost, consider the disruption to the work day caused by a brownout that causes every server to reboot in the middle of a transaction or backup procedure.
Workers will get irritated and thus their productivity is likely to be further reduced. Isn't it worth spending three or four thousand dollars to protect your investment?
With regard to return on investment, I'm tempted to give every machine five stars simply because whichever one you choose, your UPS is vital insurance for your equipment and data -- I cannot stress this enough.
How big is yours?
UPS power ratings are given in terms of Volt Amps (VA). To determine the size you need simply multiply the supply voltage (probably 240V) by the current drawn by the equipment needing protection. For example, four computers drawing 830mA from a 240V supply require an 800VA UPS (0.83A x 240V x 4 = 796.8VA).
Battery run-time is dependent on the load. If the UPS is only supporting half of its rated load it may operate for up to three times longer before losing power. Typically, UPS specifications will claim about 15 minutes runtime on half load and only about five minutes on full load.
Realistically, the load will probably be somewhere in between -- you don't want to push the limits of the UPS (and you should allow for expansion of your current needs), and you don't want to pay for an excessively large UPS. Should you, by chance, overload your UPS you'll know about it, because it will sound an alarm and flash one of its LEDs frantically at you.
Often, additional external battery packs are available which can be used to expand system capacity without totally replacing the UPS (more later). This is particularly useful in situations where it is not practical to shut down all systems with the typical five to 10 minute window.
It is also generally a good idea to avoid running a laser printer via a UPS. Such printers are extremely power hungry and thus greatly increase the size requirements of your UPS. Energy guzzlers can always be protected by a separate line filter (without battery backup).
