UPS Runtime Formula


How to manually estimate UPS runtime based on server load and battery capacity.


Use this four-step process to estimate UPS runtime.
  1. Identify amp hours (AH).
  2. Determine DC amperage for existing load.
  3. Create a base ratio from available AH.  Create a second ratio from the DC Amperage estimate. Cross multiply both ratios.
  4. Apply power-factor multiple for final conservative runtime estimate. 
N.B., This process is automated with my UPS runtime calculator and spreadsheet tool:

Identify Capacity (i.e., Total Amp Hours):   

Internal batteries usually connect in a series.  That means that the voltage increases and the amperage remains the same.  In most situations, we only need the AH from one internal battery (in a series of two batteries) -not both.  For example:

     2 x 12V18AH batteries (i.e., series) =  18 AH.

N.B., batteries that connect in parallel doubles the AH while the voltage remains consistent.  Parallel connections are common in external battery packs.

Determine Load (i.e., DC Amperage):

Load describes power consumption.  Determine the active load based on amps consumed per hour (AH).  Use the standard power formula to calculate amp use:

   W = V x A.

For example, assume that a HP server uses 300W at any given time.  We're calculating battery capacity so use battery voltage (e.g., 24V) -not 120V.  N.B., It's OK to use server-manufacturer estimates.  It's better to take measurements from UPS or server management tools.

300W = 24V x A

Runtime Calculation:

Runtime uses a ratio of capacity over load.

Capacity:   18AH
Load:         12.5A

Therefore, our initial runtime estimate is 1.44 Hours or 86.4 minutes.

Power Factor

Our initial runtime estimate does not account for power factor, diversity factors, efficiency loss, etc. We must, therefore, multiply our initial runtime estimate with a conservative power factor (e.g., .70):

Original runtime estimate:  41.6 minutes

Power Factor: .7

86.4 minutes x .7 = 60.2 minutes.
 or 1.44 Hours x .7 = 1.008 hours.

That's it!


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