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!

UPS Runtime Tool


Ensure UPS unit has sufficient capacity, voltage, and runtime.  This online tool calculates UPS runtime based on server load and UPS capacity.


Edit the AH, Watts, and DC voltage fields for a custom UPS runtime estimate.

Runtime formulas and additional information located at:

Watts-to-Amps Tool


Tool that coverts electric power from watts (W) to current in amps (A).  How to choose the correct UPS based on server load.  Ensure backup UPS unit has the correct voltage and amps for runtime.


W = V x A.  Use the voltage from the UPS batteries -not 120V!

Use the Watts-to-Amps tool to calculate Amps/runtime based on server load/Watts:

Fix Runtime and Calibration Issues

How to Fix Premature UPS Shutdowns.  


How to calibrate UPS units.  How to achieve the best runtime.  How to prevent early shutdowns.   Smart-UPS units use Battery Constant to calculate runtime.  Inaccurate battery constant leads to inaccurate runtime calculations (i.e., drifting).


UPS reports inaccurate battery runtimes.  Poorly calculated runtime cause premature shutdowns during power outages.  Misconfigured external battery counts or old firmware can also trigger early shutdowns.


Batteries become less efficient as they age.  The UPS changes its battery constant value to reflect these inefficiencies over time.  The UPS unit is supposed to reset its battery constant after installing new batteries.

Unwelcome drifting occurs if the UPS does not detect new batteries nor reset its battery constant value.  As such, it miscalculates available runtime that leads to premature shutdowns.  According to APC, drifting may occur when factory batteries are replaced with generic brands.  Drifting may also occur during a hot-swap replacement.  Self-tests may also introduce drifting.

Soft Runtime Calibration: 

Initiate a runtime calibration from the management card.  This process recalculates runtime capacity based on its current load.  Run "Soft" Runtime Calibration from the network management card (NMC).

Calibration Tips:  

  • Do not run calibration on new UPS units. 
  • Re-calibrate after replacing old batteries with generic batteries. 
  • Schedule an annual calibration.  
  • Discharge and recharge old batteries -before calibration.   
  • Calibrate with a 40% load. 
  • Completely discharge and completely recharge UPS.
  • Change/ reset the number of external batteries from the management card.

Manually configure the battery constant if the soft runtime calibration fails.

Hard-set the Battery Constant:

Change the battery constant value from the serial console -otherwise known as the nuclear option!  Use caution because incorrect commands can brick the UPS.
  1.  Shut down UPS.
  2.  Remove NMC.
  3.  Use an APC serial cable to console to the UPS:
         2400 baud,
         uncheck FIFO.
  4.  Enter Smart mode:  Shift+Y (i.e., CAP "Y").
       Prompt changes to SM#.
  5.  Enter Program mode: Y, 1, 1
      N.B., each comma represents a pause. 
  6.  Enter "0" to display the battery constant.  The battery constant value (e.g., B2) is located under column 0 and is in HEX format:

    UPS Model  4   5   6   0   Hex   Firmware
    750XL          EE  F8  B2
  7.  Change the battery constant value to FF.
         Use "+" or "-" keys to change the value.  
  8. Type "R" to save and exit.
  The FF value is the highest possible battery constant value.  It is entirely inaccurate -don't panic.  Our goal is to maximize runtime and eliminate premature shutdowns.  The UPS will still initiate a shutdown when battery voltage is less than 10.5V per battery.

Use manual calibration method after the battery constant is rest.

Manual/ Hard Calibration

   Hard calibration provides the most accurate runtime estimates.  This method generally corrects inaccurate soft calibration estimates:
  1. Move production servers to an alternate power source.
  2. This test requires at least 30% load.  Attach old servers, workstations, etc to simulate production load.
  3. Ensure batteries hold a full charge.  
  4. Pull the plug so that the UPS operates on battery power.  
  5. Allow the UPS to operate until it shuts down.
  6. Plug the UPS back in and allow it to recharge.
The UPS now provides an accurate runtime estimate.  It is ready to protect production equipment.

That's It!

UPS External Batteries


   How to calculate and configure external battery count for UPS units.

Battery Packs:

   UPS units can run for days with a sufficient number of external batteries -not so for some APC units.  The UPS uses battery count for runtime calculations.  For example, some UPS models cap the number of external batteries.  In other words, runtime won't benefit from additional batteries that exceed this cap.

   UPS units use external battery count for runtime calculation.  This process provides maximum runtime and prevents premature shutdowns.  Configure external batteries in the NMC GUI.

   At first glance this process seems simple enough.  However, determining the correct number of external batteries gets confusing.  For example, does it want a count of all physical batteries?  Or does it want a count of battery packs?

   In most situations, battery count equals the number of physical batteries each battery pack.  N.B., this rule only applies so long as the internal and external batteries are identical.  For example:
  •  750XL UPS uses two 12V18AH batteries.  
  • SUA24XLBP battery pack uses four 12V18AH batteries. 
  • The network management card (NMC) external battery count equals four.
   However, there are situation when internal and external batteries are different.  Consider the following:
  • 750XL UPS uses two 12V18AH batteries.
     ~18AH per battery; or a total of ~36AH.  
  • UXBP24 battery pack uses four 12V75A.
     ~75AH per battery; or ~300AH total.  
   In this situation, the external battery count is not four.  Rather, it equals 16.  Huh?

   In both examples, the external battery count is the total sum of Amp Hours (AH) of all external batteries; divided by the AH of ONE internal battery.

     AH for total external batteries:  300AH
     AH for single internal battery:    18AH

               Figure 2.  Calculating External Battery Count 
  The battery amperage is usually listed on the battery.  Alternately, look up the UPS or battery pack model at  

Console Control:

   Alternately, use the console (i.e., not the NMC) to set the battery count:
  1.  Shut down UPS.
  2.  Remove NMC.
  3.  Use an APC serial cable to console to the UPS:
         2400 baud,
         uncheck FIFO.
  4.  Enter Smart mode:
       Press Shift+Y (i.e., CAP "Y").
       Prompt changes to SM#.
  5.  Enter Program mode: Y, 1, 1
      N.B., each comma represents a pause. 
  6. Use the "+" or "-" to change the battery count.  
  7. Type "R" to save and exit
That's It!

Electrical Power Planning for Server Rooms: Best Practices

Last updated  November 11th, 2013 by Steven Jordan.

Server room power requirements:  Strategy for electrical circuits and outlets.

     What's so complicated about power?  Plug in the machine and watch the LED lights come to life.  Unfortunately power supply planning is a bit more involved.  Larger server rooms typically have 110V and 220V outlets.  To simplify the process we'll examine typical 110V outlets and how they supply appropriate power to the sever room.

     Assessment Planning:  Layer 1 may not be glamorous but it is the essential foundation for all server room dependencies.  Don't assume that server room outlets can support every device you can throw at it.  Likewise, it's poor practice to substitute power strips when there appears to be a shortage of electrical outlets.  An assessment of available electric resources will avoid these pitfalls.

    Visio.  Begin the assessment by conceptualizing the server room into a Microsoft Visio diagram.  The diagram does not have to be to scale.  We're simply mapping electrical outlets and basic server locations.  This is an example of a server room circuit map I had previously created:

N.B., 110 outlets are represented with a singular power symbol.  220 outlets are represented with the power symbol and with "240V".  The 240V stencil is nominal for either 208 or 240 Volts.  APC UPS units, however, can be configured with inputs and outputs of nominal 220, 230, and 240 voltages.  Like wise, 110V usually refers to nominal 120V. 
      Power Formula.  Visio mapped the 110V and 220V outlets in the server room.  Each outlet should then be assessed to ensure a sufficient supply of electricity.  On average, 110V outlets can support approximately 1440 Watts, or 1.4kW.  The standard power formula is used to estimate an appropriate devices-to-power ratio:

     Electrical power is the measurement of output.  Server consumption of electricity is ratted in Watts (W) (CDX, 2013).  The power formula measures consumption:  

P = I x E 
P = Power, I = Current, E = Volts  

     Use the power formula for server room electricity consumption:

W=A x V

W = Watts, A = Amps, V = Volts     

     Why is this formula important?  Consider a 110V run provides a nominal 120V to each outlet.  The maximum theoretical output is 1800W:

1800W = 15A  x 120V

     Do not cross the 1800W threshold!  Design load should be approximately 80% for the maximum output to prevent overloading (power outages):

1440W = (15A x 120V) .08

     The design load is only an estimation and does not account for every situation.  Circuit breakers do not limit Watts; rather Amps.  Consider a single device can draw more than 15A (i.e., don't use toasters or hair dryers in the server room).  It is however, important to determine reasonable consumption for all devices in the server room.

    Excel Spreadsheet.  Use an Excel worksheet to document each server and its average power consumption.  Columns represent individual circuits.  Rows represent miscellaneous servers and devices.  Average power consumption is then recorded to corresponding cells as Watts.  Precise measurements are preferable but not always be available. Use rough estimates in lieu of recorded statistics.  Remember, estimates can be updated to reflect closer approximation at a later date.  Armed with this information, Excel formulas can determine total output for each circuit in the server room.  

    There are a number of methods to estimate power consumption.  Amazon sells a variety of tools to determine electrical load.  A popular (and low-tech) device is called the kill-a-watt and sells for $30:

     Server tools may provide additional power consumption data.  Dell's IDRAC management card provides accurate PSU data.  For example, the Dell Open Management Server Administrator (OMSA) determined our Power Edge R420 used on average, 66 Watts. 

Dell also provides a power assessment tool at their website:

     I found the online estimates useful, albeit overly conservative.  According to Dell, the same R420 server was expected to consume an average of 224.2 Watts.  That's a 158W difference of capacity planning!  There is an important distinction between maximum rating and actual consumption.  In reality, most servers never experience more than 300W at any given time.  It should also be noted that data for Amps, as well as Watts are provided.  

     APC UPS management cards may also provide power consumption data.  The following UPS log shows the output for Voltages and Watts (as a percentage):
     How can this data be used to estimate appropriate circuit load?  The maximum rating for the Smart-UPS 1500 is 1050W (available on APC's website).  Quick math determines actual consumption:

1050W x .37 = 388.5W

    Total consumption from this UPS averages 388.5W.  Our maximum target per circuit is 1440W.  Data indicates we can tipple the current load without crossing over the 1440W threshold.

    The logs provide additional information to help our assessment.  Recall the power formula?

W = A x V       
     We can determine the average Amps:

388.5 = A x 118
388.5 / 118 = 3.29A

     What is the significance of 3.29A?  We've already determined we can add additional devices to the circuit.   Additional devices will increase current which can risk overloading the 20A breaker.  Exercise caution and incorporate a cushion for growth.  Server upgrades, including memory, CPUs, additional virtual machines, etc.. will increase the overall electrical load.    

     Electrician's Help.  An electrician can be of substantial help through the assessment process.  I recommend an electrician's help to ensure the server room has a 1:1 ratio of circuits-to-outlets.  An unexpected power outage is a particularly harsh way to discover your design only allows half of its intended power.  Comparatively, the 220V outlet with a 40A breaker is robust and forgiving.

     A qualified electrician can install 220V outlets.  The disadvantage to 220V outlets is that existing UPS units and servers most likely use NEMA 5-15 and C13 power cables.  Change of outlets requires a change of UPS units.  Consider this is a one time expense.  The 220V UPS can be paired with a step-down transformer to allow a 110V PSUs (server) to connect.

     An electrician should also review of the breaker box to determine the main amperage rating.  The main amperage rating is different from the ratings of individual circuits.  Several circuits that run near capacity can trip the the main (DIY, 2013).   The electrician can also ensure 20A breakers are used for the 110 runs.  Most importantly, if the circuit breakers are not clearly identified, have the electrician label all breakers and outlets.  Electricians can be expensive but power mitigation in priceless.

    Planning for appropriate server room power is the precursor to assessing uninterrupted backup power supplies...   


Power Supply for APC AP9600 Chasis

Last updated  November 14th, 2013 by Steven Jordan

Issue:  APC AP9505 power supply for the APC AP9600.

Our branch office had an APC UPS without a slot for a management card.  I purchased an AP9600 management card chassis.  The chassis provides a slot for the  management card and uses a serial cable to communicate with the UPS.  Dell sells the chassis for roughly $100.  

I was surprised to discover APC does not include a power cable.  I figured the power supply wouldn't cost too much.  I looked up the power supply number, AP9505, and found the cable cost just as much as the chassis! 

I could not find any documentation on the power supply specs so I ate the extra $100.  For reference, the AP9505 consists of the following: 


Brand:  CUI Stack
Model:  DV-2440 AC Adapter
Input:  120VAC 60Hz 16W
Output:  24VAC 400mA