Bowie Bailey Bowie_Bailey@BUC.com wrote:
If you don't require a long runtime, then you don't need to get a huge UPS. APC's website has a calculator that can help you determine which UPS will work best based on your equipment and desired runtime.
However, give yourself some leeway to allow not only for changes in your usage pattern (addition of hardware or changing hardware) but also degradation of the battery over time.
I've generally found that if you're pushing a UPS past 70-85% capacity, you may not (depending on the system) have sufficient runtime to shut down completely before exhausting the battery. Other things to take into account is if you have programs that take a while to shut down (some databases can take a *very* long time) and whether or not you have to power auxilliary equipment such as monitor during an outage.
I usually like to have a UPS loaded to only 50%. That gives a decent enough battery time for either allowing an extended shutdown time or for carrying the system through multiple short failures without bringing the system down at all.
There is one flaw that I know of with APC brand UPSes, although I wouldn't be surprised if other UPSes are similar (since APC has traditionally set the standard in the market): There is a small window between the time that the UPS initiates a shutdown and the control software (such as apcupsd) tells the UPS to kill power in (some number of) seconds. If your mains power returns and stays on during that window, your machine may not restart by itself: It may be too late to cancel the OS shutdown and subsequent power-off, but because the UPS still has mains power it doesn't trigger the power cycle that allows the computer to reboot (assuming you've configured your BIOS to boot at power-on). In my experience this doesn't happen often (I've seen it probably twice in 15 years), but it does happen and can be a real PITA if you don't have anyone on site that can deal with it.
Devin