How to Select a Battery Backup for Your Computer

If you’re looking to be very conservative in your power estimation needs, go with the maximum rating of the PSU and peripherals (this way you’ll end up with extra battery life instead of too little battery life). Alternatively, you can increase the precision of your calculations by using a measuring device and allocate more of your budget towards UPS unit features you want and less towards buying a bigger battery.
Regardless of whether you use the less precise or more precise method, you now have a wattage value. For our calculation examples we’re going to use 400w as our value.
A simple rule-of-thumb calculation you can use to determine how much UPS is as follows:

1.6 * Wattage Load = Minimum Volt-Amperes (VA)

Volt-Amperes are the standard measurement used to describe the capacity of UPS units. Using the equation above we see that the minimum VA rating we’d want for our 400w needs would be a 640 VA rated system.
Now that we have a minimum rating, the next question on everyone’s mind is most likely: How long will that minimum system run the setup? After all you’re getting a battery backup system for your computer to keep everything running when the power is out.
Unfortunately there isn’t a super quick rule-of-thumb calculation for determining the runtime like there is for determining the necessary minimum VA. We’ll show you how to do the calculation so that you can double check manufacturer estimates if you wish, but it’s not a particular speedy or fun calculation to run through.
In order to calculate the estimate runtime of a UPS system you need to know four things: the VA rating of the UPS unit, the number of battery cells in the UPS unit, the DV voltage rating of those batteries, the capacity (or Ampere-Hours) of the batteries. Unfortunately not all of this information is easily cribbed off of a simple web site listing for the product you’re interested in running the calculation on so you’ll likely need to dig through a PDF of the manual, supplemental documents, and so on to get it all. In other words, it’s a huge pain.


Once you have all the requisite information, however, you can plug it into the following equation:

( Battery Voltage * AH Rating * Efficiency) / VA Rating =  X
X * 60 = Estimated Run Time in Minutes

So let’s pretend we’re looking at a theoretical UPS unit that has the following specs:

VA Rating: 700
Battery Voltage: 12
AH: 9
Efficiency: 0.9 (90%)

When we plug those numbers into the equation we get the following:

( 12 * 9 * 0.9 ) / 700 = 0.139
0.139 * 60 = 8.3 minutes

So this particular UPS unit could sustain the system under full load for a little over 8 minutes (longer if the system is under partial load).
We shared this exercise with you so that you can do the calculations if you wish but it’s not something we particularly recommend. It’s such an enormous hassle to dig up all the information (especially the efficiency rating); it’s much more expedient to use manufacturer estimate tables (which we’ve found to be on the conservative side anyways). You can check out the calculation/selection tools of the more popular UPS unit manufacturers here:




Thanks to:
http://www.howtogeek.com/161479/how-to-select-a-battery-backup-for-your-computer/