Calculating required UPS size

Yalla-One · 10-15-2011, 03:59 PM

Hi,

I currently have an APC 2200VA UPS that is beginning to cause more trouble than safety, and thus needs replacing.

With all the required equipment connected, apcups reports that it is currently under 13% load.

Is it then safe to assume that the current load of a NAS and a server and a few switches, Wireless and a gateway router collectively don't pull more than 286VA (total capacity 2200VA * 13%) and consequently that the 2200VA solution is overkill and a 1500VA solution will still be only 20% load (286VA / 1500) ?

And if I'm right so far, is it also correct that the time the solution will run on batteries will decrease maximum by 45%, so when it presently runs for 1 hour on battery, it should at least run 30 minutes with a 1500VA solution?

Google seems to present so many opinions on how to calculate this that the more I google, the more confused I get..

Anyone feeling like taking a shot, and perhaps also explaining why I'm right or dead wrong in my calculations?

Thanks in advance!

-y1

weibullguy · 10-15-2011, 05:54 PM

Quote:

Originally Posted by Yalla-One

With all the required equipment connected, apcups reports that it is currently under 13% load.

I wouldn't necessarily trust this outright. The best thing would be to directly measure the current draw of the equipment connected to the UPS. Multiply the current (in amperes) by the voltage (230V in Norway, I think) and this will give you the apparent power (S) measured in volt-amps (VA) being consumed by the equipment. If the 13% (286VA) is correct, all your equipment is drawing about 1.25A.

If you can't measure directly, you can look at the specs for each piece of equipment and get either the VA rating or the current rating. If you can only get the current rating, multiply by 230 to get the VA rating. Sum these together to get total VA load on the UPS.

In either case, add a safety factor of at least 10%.

As far as how long the UPS will supply the equipment in the event of a power loss, this is a function of the batteries in the UPS. Since we are dealing with DC rather than AC, we need to discuss real power (P) measured in Watts (W), not apparent power. The relationship between real power and apparent power is something called the power factor (pf). The pf is the ratio of real to apparent power (pf = P/S).

If you don't know the pf of the connected equipment, using a 0.7 pf thumb rule is reasonable. Newer equipment will have higher pf to meet environmental requirements (at least in the US). Thus, if we use pf = 0.7 and S = 286VA, the real power consumed by the equipment is P = 0.7 * 286 = 200.2 W. The UPS you are interested in should have a graph that plots runtime as a function of load. Using the calculated power consumption, you should be able to estimate the hold-up time of the UPS.

For example, this APC 2200VA UPS (don't know if it's the one you have) has a hold-up time of 185 minutes at 200W. If this were the one you are using and your hold-up time is only 60 minutes, either you need to replace the batteries or your total real load is more like 550W which would translate to 786VA (assuming a pf = 0.7) or 35% load. Which takes us back to the beginning when I said I wouldn't necessarily outright trust the 13% load being reported.

H_TeXMeX_H · 10-16-2011, 01:21 AM

I recommend that you don't calculate anything using VA, it's meaningless. Look for the Watts it provides, it should be written on the back in small print. If it provides more Watts than you need, then you're good.

Yalla-One · 10-16-2011, 09:09 AM

Thanks much for the in-depth reply!

When I add the kit connected to the UPS I get the following Watts from the end-user documentation:

Code:

QNAP TS-809Pro	85	W
Dell Optiplex	235	W
Cisco switch	15	W
Cisco router	20	W
Fibremodem	5	W
HomeServer3	5	W
Total power	365	W

So total power consumed is a mere 365W should give around 40 minutes runtime on battery power according to APCs product spec you tought me about, found here: http://www.apc.com/products/runtimeg...hartSize=large
Would this calculation leading to 40 minutes be correct?

The reason why there's not more runtime on the current 2200VA UPS is that it's about 6 years old with 6 year old batteries and isn't in very good shape - hence the need for a new one...

Thanks again,

-y1

jefro · 10-16-2011, 03:55 PM

They have battery trade in programs that would cost a lot less than a new system.

weibullguy · 10-16-2011, 05:13 PM

+1 what Jefro said.

Yes, ~40 minutes is correct for 365W load.

etech3 · 10-16-2011, 06:21 PM

another +1 on what Jefro said. Call APC and they can walk you through. Battery trade in should be all that you need.

I always used a rule of thumb on the life of batteries as:

Label unit and batteries with install date.
2-3 years under normal conditions=OK.
3-4 years old, keep an eye out for trouble.
4 years>up replace.
Use total watts and add a safety margin (at least 10%)
Pick a quiet time and pull the plug on the power and test. Of course I always do a single test on each piece of equipment with server being last after a full backup.
YMMV

Yalla-One · 10-17-2011, 12:46 AM

Thanks for the confirmation of the remaining time, which means that the 1500VA will do the trick!

Since battery trade-in programs rarely offer new bearings to reduce the fan-noise and also doesn't update the serial port to USB or reduce a massive heat generation, I'll stick with the plan to update the old one to a brand new one, but in general I'll agree with the battery change advise. In that case, it would most probably make the most economical sense to build the battery pack from brand new cells one self, or simply buying a subscription. However, in this case the batteries are just the trigger factor. The old one has to go.

-y1