The Power Experts

How to Calculate Battery Run Time

Power Solutions - (800) 876-9373

Power Solutions Tips & Information

August 2007

In This Issue

Problem Solvers

Calculating Battery Run Time



Dear Dan, 


In this month’s newsletter, I answer one of the more frequently asked questions we get at Power Solutions.

“How do I properly evaluate battery specifications for the run time of my UPS?” Along with that I’ll explain another popular query, “What impact does a UPS’s Inverter Efficiency, ambient room temperature and the batteries’ watts per cell have on a UPS’s run time?” We are dedicated to providing you reliable power quality solutions and superior service. This includes sharing information that is relevant to your business. Please feel free to call or email me to discuss how Power Solutions can help you properly size your UPS and establish a reliable power protection strategy. Sincerely, Newton Black, Power Specialist Power Solutions LLC

 Problem Solvers 


Battery Upgrade or Replacement 

Need a Battery Upgrade or Replacement for Your UPS?


Power Solutions provides upgrade options, installation and battery replacement solutions for ALL UPS products.

Whether you need a new battery for your APC, Powerware, Best, Tripplite, Liebert or Belkin UPS System we have the solution for you – usually in stock

and available for same day shipment.


If you prefer, our technicians are available to install your new batteries and properly dispose of your old batteries.


Learn More 





LORAIN eXpandable Power (LXP) SystemThis Small Power Platform is Perfect for Central Office, POP and Co-locate Sites. 


For the optimum small power platform, see what Emerson Network Power’s LORAIN® eXpandable Power (LXP) System has in store.


Rated for continuous operation in rugged environments ranging from minus-40, to plus-80 degree C environments, without compromising NEBS Level 3 compliance.

View Options 



How To Calculate Battery Run Time 



This article explains how to calculate the battery run time you should expect for a specific brand and model of UPS Battery.


For the purpose of this exercise there are some items that we need to know including the UPS inverter efficiency, allowing us to calculate the kWb, the DC endpoint or cut-off voltage and finally the average room ambient temperature therefore we will make certain assumptions.You can easily change these assumptions to suit your application.


Let’s make the following assumptions:

-Inverter efficiency 90%.(Industry norm is 90%-98%)

-End point voltage 1.67.(Typical reference used in battery specifications)

-Ambient Room Temp 77F.(Typical reference used in battery specifications)




A UPS system has one string of batteries, consisting of a quantity of 30 each, Northstar Model NSB12-475 and the rating of the UPS is 40kW.The battery has 6 cells per unit.


Step 1:

Calculate the Actual Battery Load for a 40kW UPS Load


40kw load assuming a 90% inverter efficiency is actually 44.44kw battery load


40kw (actual ups load) DIVIDED BY .90 (90% inverter efficiency) = 44.44kw (actual load on the batteries)



Step 2:

Calculate Watts Per Battery


Each battery will see 1481 watts


44.44kw (actual load on the batteries) DIVIDED BY 30 batteries = 1481 watts per battery.



Step 3:

Calculate watts per Cell


Each cell of each battery will see 247 watts


1481 watts per battery DIVIDED BY 6 (cells per unit) = 247 watts

247 watts per cell



Step 4: Estimate Your Expected Run-Time


The specifications for this battery show that at 247 Watts per cell and 1.67V your run time is between 45 and 60 minutes.


. Constant Power Discharge Ratings Chart


Power Solutions | PO Box 100 | Barrington | RI | 02806