How to calculate UPS battery backup


It is nearly impossible to know when the next power outage will occur and frankly, they can be rather damaging unless you have the tendency to save your work every 2 minutes. I remember I have been working on this Photoshop file for nearly two hours, and a power outage came and erased everything I worked on – the next day I went to the market and bought an UPS. Now, before I bore you with useless stories let us see what you can find within this article:

  • The answer to the popular question “how to calculate ups battery back-up”
  • What is an UPS (with friendly details)
  • What types of UPS you can find on the market

Introduction to the UPS battery back up

UPS (Uninterruptible Power Supply) systems have been built to compensate for those power outages and to help you save your system and your work. You might have found out by now that those sudden changes in voltages can affect your computer – it can damage your power system along with your main board and CPU. Aside from this, it can produce light damage to your hard drives and other devices you have in your computer. The bottom line is – power outages are harmful for both your work and your PC. The thing is, UPS’ have become quite useful especially nowadays when they can have up to one hour battery back-up which gives you enough time to complete your work and shut down the computer safely. However, how does a UPS work? Simple, when your power line distributes power within normal parameters, the UPS stores power, therefore whenever a power outage occurs UPSes act like a battery (similar to the back-up battery existing in laptops, only larger and based on a different principle).


Any UPS system is placed between your equipment and the power socket. When electricity flows from the socket to the UPS and then to the computer, the battery is continuously checking power levels, and collecting power at the same time. When the electricity flow is not within normal parameters anymore, the UPS will start compensating for the lack of power by connecting the batteries. Once the power flow has been restored, the UPS will start recharging its batteries again. Owning an UPS, battery back up will not only protect you from power outages, but also from changes in the power flow (lower voltage may damage your components too).

Types of UPS battery back-up

The market is full of UPS battery back-up systems and some of them have different purposes. Here are the two most popular types you can find on the market at any time:

  • Offline UPS – Offline UPS battery back-up systems work similar to laptop batteries. Once the power source is removed, they kick in and supply the needed amount of power, and once the power outage is removed, the UPS goes back to recharging its batteries. The only inconvenience here is the fact that when power is restored, the UPS temporarily closes the transfer of power between the computer and the source thus making it an unreliable choice in many cases.
  • Online UPS – This is one of the most reliable types of UPS battery back-up systems since they provide electricity at any time, without any closing down of power transfers (similar to the offline UPS). Basically, an online UPS will not only protect your computer for power outages, but also from voltage irregularities.

How to calculate ups battery backup

When I first asked this question myself, a friend of mine kindly sent me to the manual (a joke, obviously). In the meantime, I found out that calculating ups batter back up is not such a big deal as I might have thought in the first place. In order to make everything as clear as I can, I will use a small example. Let us assume the following ideas:

  • Inverter efficiency of UPS battery is 90% (the norm is somewhere between 90% and 98%)
  • Battery end point voltage 1.67 (usually specified on the UPS itself, or in its manual)
  • Environmental temperature 77F

Assuming that we know the following things about our UPS : one string of batteries, 30 each, UPS rating 40kW and 6 cells per unit, we can break down the calculations in three steps:

1.) Actual battery load for 40kW

40kW … 90% efficiency means an actual 44.4kW load

(you can calculate this by dividing the rating of the UPS by inverter efficiency – 40/0.9 = 44.4kW)

2.) Watts per Battery

44.4kw (previously calculated) will be divided by the number of batteries we have available, and the result will lead to 1481 watts per battery.

(you can calculate this using the equation: 44.4Kw/30 = 1481W)

3.) Watts per each Cell

Giving the necessary data we will deduct that each cell needs 247W.

(calculate this using the equation: 1481W / 6 (the number of cells per unit) = 247 watts per each cell)

Now we can easily estimate the run-time of your UPS battery back up system. Giving the fact that we know that each batter has about 247W per each cell, and we have a battery end point voltage of 1.67V your run time should be somewhere between 45 minutes and 60 minutes.