Power supply wattage and UPS question

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I'm trying to decide what rackable UPS to buy.
So I have a Poweredge 2850 with redundant power supplies. It says the power
supply is rated at 700W.  Does this mean because I have redundant power my
server is actually drawing 1400W while in use?  
If so, what kind of UPS should I get?  Does this mean I need a 1430W / 2200VA
UPS?  If I got a 480W UPS is that too small?  How do I know?  I'll have a
small switch and a firewall on it too.

Re: Power supply wattage and UPS question

Normally redundant means "back-up". So if you have a 700 Watt supply
for your server and a redundant one, than your using [or have] 700
watts. The other supply is off until the first one fails. If both
supplies were operating full-up ~ what is redundant?

The back of the unit should indicate the power draw. Look at the power
drain for each unit, add them, add some margin [10%] and get a unit
that supports that number.


Re: Power supply wattage and UPS question

On Mon, 02 Jan 2006 02:31:58 GMT, kenw232@yahoo.com (Ken
Williams) wrote:

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If you link to the specs, manufacturer product and parts
page it might help.  Generally speaking (considering the
class of system) it is very unlikely you have 2 x 700W PSU
in it, more likely 2 x 350W PSU.  That means the system
"could" run entirely off of one 350W PSU, thus the
redundancy aspect of it, that if one psu failed it would
keep running off the other one.

That only means the system uses LESS than the 350W the PSU
can provide.  For example, if you have a 20 gallon gas tank,
it doesn't mean you use 20 gallons driving to work,
necessarily, only that it was enough gas to get you where
you needed to go.

A random guess would be that the system uses roughly 250W, a
little more or less depending on load and # of drives.
There is further variability though, in what else needs
powered along with it, like a monitor or switch or (not).
Such is the case with servers- sometimes put on their own
supplies and other times a larger UPS network that handles
other equipment too.

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No, the power supply itself can be mostly ignored for the
purposes of factoring UPS capacity.  

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If you mean 500VA, and in a consumer-grade unit, yes that's
cutting it too close- it'll probably work, but for a server
that is too little margin.

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We can't know what your switch and firewall consume, but
typical _modern_ standalone units are relatively low power
consumers, maybe 25W total.

What is the strategy for the UPS?  Is it meant to keep
system running during an outtage?  If so, you may need a
much much larger unit.  If it's only needed to allow system
to survive momentary power "flickering" or shut off
gracefully in event of a full outtage, then all it needs do
is support the current, VA the system requires.  In this
case, a 700VA UPS would be about minimum as it's good to
factor for double the actual estimated wattage of the unit
(250W above).  On a high-end commercial UPS, there may be
less fudging involved, rather than 2 X you might get away
with 1.25-1.5X that 250W, but in the end it is a more
expensive UPS too, so either way it can be a factor of what
the budget is.  Given the other devices too, it might be
nice to aim at 1000VA and AVR (online voltage regulation).

If you have different needs for the server and for the
switch or firewall, for example if they're connected to
different equipment not all of which is succeptible to same
power grid outtages, alternate UPS or generator, etc, it
might make sense to put the switch and firewall together on
a different UPS than the server, as those two might run for
multiple times as long alone on an UPS, while the server
will not run for over an hour without the UPS becoming
costly.   Define the needed runtime per each device, weighed
against the budget for the UPS.  The generic answer for
powering all 3 is 1000VA, 700VA if budget is tight for them
to run long enough to shut down gracefully, less than 1/2
hour runtime.

Re: Power supply wattage and UPS question

On Mon, 02 Jan 2006 02:31:58 GMT, kenw232@yahoo.com (Ken Williams)

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You need to determine actual power draw and the run time you need.
You can determine maximal & minimal watts from specs & use that to
calculate your runtime needs.

Or a handy tool to have anyway is the Kill A WATT meter

(Or something similar).  The advantage is you can quickly & easily see
what the total draw really is on startup, idle, & load, and "average
use".  This is faster & potentially more accurate.  With a push of a
button you can get real VA (which is how UPSs are typically sold).

You won't use it very long to make this purchase decision, but it is
an inexpensive tool that can be quite handy to have around (IMHO).

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