UPS question

Michael Powell nightrecon at
Thu Aug 12 07:45:46 UTC 2010

Oliver Fromme wrote:

> Ryan Coleman <ryan.coleman at> wrote:
>  > He thinks that at 500W needed it would give me about 12 minutes on
>  > a 1400VA.
> That W and VA numbers of the UPS are pretty much irrelevant,
> because they tell nothing about the capacity of the battery.
> Those numbers only give an upper limit on the power that
> the UPS can handle (i.e. you cannot connect devices totalling
> 800 W to a 500 W UPS, for example).
> In order to be able to estimate how long the UPS can power
> wattage, you need to know the capacity of the battery.
> The capacity is usually given in Ah units (Ampere hours).
> For example, a battery with 10 Ah capacity can deliver
> 10 Ampere for 1 hour, or 20 Ampere for 30 minutes, or
> 30 Ampere for 20 Minutes ...  and so on.
> At a typical battery voltage of 12 V, 30 A would be 360 W.
> So, theoretically a 10 Ah battery would be able to hold
> devices that use 360 W for about 20 Minutes.  In practice
> it will be less because no UPS has 100% efficiency.
> Best regards
>    Oliver

Another often overlooked detail is how long the battery will last. These 
amp-hour figures are all for new batteries, and the number of 
discharge/charge cycles has some effect over time as well. Generally 
speaking when a UPS just sits there and does very little the batteries are 
like new for the first two years. Somewhere into year 3 they begin to nose 
over the derating curve. So at year 3.75 they will have signifigantly less 
full power runtime than when new. The quality of manufacture for the 
batteries controls this, for example with lead-acid how much metal goes into 
the plates.

I admit to being bitten a time or two: There is a certain tendency to put 
the UPS in the rack and walk away and forget all about it. I've learned the 
hard way to keep records so I can replace weak batteries in a timely 
fashion. Or this happens: "But that server should have been able to stay up 
20 minutes instead of crashing at 7 minutes..." 


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