Anyone have any tips on choosing a UPS? Here's what I'm doing now:

I add up total power supply wattage stats of machines to be plugged in to the UPS and divide that by the output capacity wattage of the UPS. Based on that, I try to keep the result no higher than .60. If it's higher, time to get another UPS.

Is this a meaningful calculation?

Thanks for any links or advice.

Keep in mind, while a PC power supply is rated at (for example) 300 watts, very rarely will the machine actually draw that much power. Perhaps on initial power-up (when drives are spinning up etc) it might come close, but in general you won't be drawing nearly that amount.

On my home network, I have an APC 650 (400 watt sustained output, 650 short-term peak) running 3 mid-tower PCs, a switch, cable modem, and wireless access point. Far more than it should be powering, but it's been going strong for 2 years, and this is just a cheap home-version UPS. I've had outages of 20-30 minutes with no interruptions...

Not that I recommend doing this in a production environment, of course, but I would say you could probably squeeze a little more out of them. If you use your 60% rule, in reality you may only be utilizing 10-20% of the UPS's capacity... Just make sure you go by the sustained output rating, not the big numbers on the front (reminds me of speakers/amps, always rating "peak" power, which is really more a marketing unit of measurement).

Something else to consider, though, would be to keep the machines powered in such a way that one UPS failure can't take your whole operation out. If you have several switches/routers, make sure they are independantly powered from each other, and things like that. UPSs do fail, of course depending on the quality (and ultimately the price) of the UPS...

Why not try APC UPS Selector?

APC UPS Selector URL (http://www.apcc.com/template/size/apc/)