The Power Experts

How to Calculate Battery Run Time

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Power Solutions Tips & Information

August 2007

In This Issue

Problem Solvers

Calculating Battery Run Time



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Common Power

Selecting the
Correct UPS

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.


Newton Black, Power Specialist

Power Solutions LLC

 Problem Solvers 






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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

-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




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)
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.




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