UPC Calculation Help

Posted on 2005-03-07
Medium Priority
Last Modified: 2011-04-14
Hi All,

I need someone to confirm my UPS calculations for threee PCs on two diffrent batteries:

I have 3 UPSs (1400VA) each with either 20Ah or 26Ah batteries (each will take 2 batteries)

I have done the following calculations (I HAVE ROUNDED OFF):

1) PC 1
1230 Watts - 6Amps (Consumption)

1230*.7 = 860VA min needed

860VA/220V = 3.9Amps usage
20Ah Battery: 20Ah / 3.9 Amps ~ 5Hours
26Ah Battery: 26Ah / 3.9 Amps ~ 6.5Hours

Now if run on a 1400VA UPS: 1400/860 = 1.62
20Ah Battery: 5Hours * 1.62 ~ 8.3 Hours
26Ah Battery: 6.5Hours * 1.62 ~ 10.5 Hours

2) PC2 & 3

500Watt Power Supply - 2.3Amps
350Watt 21" Monitor    - 1.6Amps
850Watts - 3.9Amps

850*.7 ~ 600VA min needed

600VA/220V ~ 2.8Amps usage
20Ah Battery: 20Ah / 2.8 Amps ~ 7Hours
26Ah Battery: 26Ah / 2.8 Amps ~ 9Hours

Now if run on a 1400VA UPS: 1400/600 = 2.3
20Ah Battery: 7Hours * 2.3 ~ 16.1 Hours
26Ah Battery: 9Hours * 2.3 ~ 20.7 Hours

I'm about to spend the money on the batteries so I just need to know that I didn't make any mistakes.


Question by:Pete2003
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LVL 69

Accepted Solution

Callandor earned 1600 total points
ID: 13476055
You did the inverse on the watts/VA calculation: divide watts by 0.625 to get VA.  See the table here to get an idea of runtime under various loads: http://www.apcc.com/products/runtime_for_extendedruntime.cfm?upsfamily=21

Author Comment

ID: 13476177
Ok ... but then are my calculations wrong .. if so where ...
LVL 69

Expert Comment

ID: 13476674
> 1230*.7 = 860VA min needed

should be

1230/0.7 = 1757 VA needed

Your actual runtimes will vary, because the amount of current available will vary, depending on the load.  For example, running very close to the rated capacity will give you less runtime than you think, because of the higher current draw.  See the diagrams here for an explanation: http://www.batteryuniversity.com/partone-16a.htm  The best indicator of runtime will be the posted times at the manufacturer's website for different loads, which is in the first link I posted.
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Author Comment

ID: 13476817
ok will look ... but my UPSs are no name brands ...

Author Comment

ID: 13476838
Also I have a problem here ...

The 1st PC is actually 2 PCs, ADSL modem , hub and a monitor ...

300Watt - PC1    (1.4Amps)
300Watt - PC2    (1.4Amps)

220Watt - ADSL   (1Amp)
110Watt - Router (0.5Amps)

300Watt - Monitor (1.4Amps)

So by those calculations a 1400 VA UPS wan't be able to handle that ... I thought they shoudl be a ble to run a fe w PCs ...
LVL 69

Expert Comment

ID: 13477330
Get another UPS to offload some of the devices, or get a 2200 VA UPS.  ebay has them for sale all the time.

Author Comment

ID: 13479560
ok .. but do you agree that the 1400VA will not handle the devices listed above ?
LVL 69

Expert Comment

ID: 13479604
Yes, that's why I made my last recommendation.

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