Which UPS for two server?

Gordon Henderson gordon at drogon.net
Thu Dec 21 03:56:36 CST 2006


On Thu, 21 Dec 2006, Alessandro FAGLIA wrote:

> My question is: which is the best (in terms of available power)
> rack-mounting UPS that I can buy to serve these two babies?
> If the total power is 1420W, I could go for the APC Smart-UPS 3000i,
> 2700 watt (probably the APC Smart-UPS 2200i, 1600 watt is enough, but
> the load will be tight to 100%). What about the current spike I
> mentioned before?

The general rule I've found to hold true for APC units (without extended 
range batteries) is that they'll last 15 minutes on half load and 5 
minutes on full load. (it's more or less linear when the load is half or 
less) This seems to approximate for other UPSs too - eg. I use MGE units 
in my office and they work out at about the same.

So I'd go for the 3000i if you can afford it, but it all depends on what 
it is that you're trying to protect against, and only you'll have that 
knowledge - is the power company in your area generally reliable, or not? 
Do you get "brown-outs" when too many people turn something on (eg. 
heaters or AC units) and the power company takes a few seconds to respond, 
and so on. Where I am (UK) the power is generally reliable, but when it 
goes out, it tends to stay out for a longer time than my UPSs can manage 
)-:

Also remember that watts != VA. (but I think you know this from the 
paragraph above) UPSs quote VA because it's a higher number, so looks 
better on the sales sheet. The actual conversion factor from W to VA 
varies, depending on the type of load. For purely resistive loads (lights, 
heaters) then it's 1:1 but for things with weird switching PSUs (like 
computers) it's less.

You'll also find that the servers use a lot less electrickery than quoted 
- they might peak at that during start-up, but Dell (and most 
manufacturers) want to quote worst-case figures. (in one place, I have 2 x 
2950's on an APC 2200 unit and it only just registers 1/5 of total load)

Gordon



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