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)

Example:
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.