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Power Calculation for UPS Selection

Power calculations is approx as below example:
- 2 x Billion 7300GX modems 2 x (12V * 1A) = 2 x (12W) = 24W
- 1 x Linksys RV082 router (240V * 0.2A = 48W)
- 1 x 24 port HP Procurve 2524 switch (approx 36W)
- 1 x Dell PowerEdge 2850 with 2 x CPU's and 4 x hard drives (700W max)
TOTAL = 808W

Because 1.4VA = 1W (approx), those devices would consume 808*1.4 = 1131VA
Support using a 3000VA UPS, it can last for 2.6 hours (approx 3000/1131)
With a 5000VA UPS, it can last around 5000/1131 = 4.4 hours

In actual case, with respect to run time, it is not the 3000VA or 1000VA that determines your runtime – its the capacity of the batteries. You can get a 3000VA UPS with quite a small battery (= short runtime) and you can get 1500VA UPS with additional plug-in batteries for super long runtime. For example, a second hand 3000VA UPS from ebay might have the ability to support 15A of load but the shagged out batteries may give you less than 5mins of runtime. Hence, you have better check the UPS configuration from vendor web site.

3KVA is the maximum load the UPS can handle.
The AMP HOUR rating of the battery system is what determines runtime.
eg a 12V battery with a 7AH rating will run a 12V 1A load for 7 hours, or a 12V 2A load for 3.5 hours.
7AH/1A = 7 Hours
7AH/2A = 3.5 Hours
Our 12 V Sealed Lead Acid (SLA) 7AH battery has an energy density of 12V at 7Amps for 1 hour = 84Watt Hours = 5040 Joules.
except that's not quite correct because of battery aging.
In most cases, it would recommend a max of 2 hrs for commercial installs. This is because AC and lights, phone system are also off, resulting in H&S issues. For home use, it would recommend 30 min max.

Runtime Example from UPS vendor web site: