The first factor is load size--how much you plan to power with it, you have to get a unit that is sufficient--thus the VA rating.
The next biggest factor most people consider is load time--which is just running the math against the battery size and your equipment power requirements. Bigger batteries cost more yet need replacing at the same frequency. There is also physical size and form factor: - server-room appliance type--these look like a giant cabinet along the walls of your colo. Leibert is one common brand. - rack mount units, 2U on up (never seen a 1U though they probably exist), often 5U - tower units - tower units with modular extra battery packs - all-in-one power-cord units like little APCs that look like an enlarged power strip Any expensive unit will have hot-swappable batteries. Some smaller/cheaper units do not, some may not even let you replace them at all, let alone hot. There are basically two types: - double-conversion/aka full-online - single-conversion Most office-targeted UPSes (eg APC, Tripp Lite, Belkin--if they make one), will be single-conversion. This means that wall power is transferred straight through to the load until it fails, at which point there will be a (usually) negligible switching time before it can turn on the inverter and continue powering the load. The combination of capacitance in the unit and often capacitance in your device's input power supplies usually suffices to ride out the switching time--however some power supplies may glitch on their voltage rails when this occurs. A few rare devices may get power cycled abruptly. More expensive units are double-conversion. This means that they are always producing power from their inverter such that your equipment will never experience any switching time when the UPS loses wall power. This costs more energy to do--notice that previously in the thread one unit mentioned drew 100W minimum. More energy also means producing heat. IME, most single-conversion units don't produce noticeable heat except for when the charging circuit is active or it is actively powering the load (loss of wall power). Software connectivity may be a factor. Some have ethernet ports, others serial ports, some nothing at all. Probably lastly to consider is noise. A better design means it won't heat as much and needs to run its fans less often. Longevity and design quality are of course important. Too bad these factors can't be read as numbers on the spec sheets... :) /* PLUG: http://plug.org, #utah on irc.freenode.net Unsubscribe: http://plug.org/mailman/options/plug Don't fear the penguin. */
