How to Estimate UPS Runtime

By Turjo, Gaea News Network
Thursday, November 11, 2010

It is nearly impossible to know when the next power outage will occur and frankly, they can be rather damaging unless you have the tendency to save your work every 2 minutes. For those who don’t like to save their work every 2 minutes, using a UPS (Uninterruptible Power Supply) is the ultimate solution to power outage problem.

But the question is, how to know how much time your trusted UPS will give you before it runs out of power? Here is quick glance at how you can also calculate how much time your UPS battery will provide backup.

  1. Determine the efficiency or “power factor” for your UPS’s DC-AC converter. This information is normally found in the “Technical Specifications” section of the UPS-enabled device’s Owner’s Manual. If you can’t find the efficiency there, try searching the manufacturer’s website or send on inquiry to their customer service email address.
  2. Determine the capacity (in ampere-hours or “AH”) for each of the UPS’s backup batteries. To approach the voltage levels of an electrical outlet, most UPS models will link several 12-volt, sealed lead-acid batteries in a series. Some manufacturer’s will include the capacity of each battery in the Owner’s Manual under “Specifications.” If you cannot find the battery capacity listed in any of the UPS’s literature, you’ll have to carefully remove one of the panels from the UPS’s housing to see the capacity printed on the batteries themselves.
  3. Add together the voltages from all of the batteries from Step 2. This will give you the combined DC voltage output for the UPS. Note: do not add together the capacities, as that value in unaffected by the series circuit configuration.
  4. Multiply the combined DC voltage (from Step 3) by the UPS’s power factor (from Step 1). This result will represent the effective AC voltage output by the UPS.
  5. Determine the voltage and amperage ratings for the appliance that the UPS will be backing up. These values are typically found either printed on the back of the appliance or published in the “Technical Specifications” section of the Owner’s Manual. As a last resort, you can search of this data in the “Products” section of the manufacturer’s website.
  6. Multiply the appliance’s voltage (from Step 5) by its amperage (from Step 5) to calculate its power consumption in volt-amperes (VA).
  7. Divide the appliance’s power consumption (from Step 6) by the UPS’s AC output voltage (from Step 4). This will give you the current (in amperes) the appliance will draw from the UPS.
  8. Divide the single battery capacity (from Step 1) by the current draw (from Step 7). This will give you the ideal maximum backup time for the UPS, in hours.

For Example :
For any particular UPS which has the following measurements,

UPS Rating = 1500VA
No. of Batteries = 2
Battery voltage = 12V
Battery rating = 17AH

UPS Back up time (in Hrs) = ( DC voltage of battery x AH rating of battery x Efficiency) / VA

Assuming that efficiency of UPS is 90%, the back up time for that particular UPS is -
= ( 2 x12 x 17 x 0.9 ) / 1500 = 0.24 hours

So the UPS is going to provide power backup for near about 15 minutes.

Hope you have found this article useful enough. Do write to us if you need more information regarding this topic.

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