How to Calculate UPS Backup Time?

Preface

After we purchase a UPS and use it, we will definitely want to know how long the UPS can provide for the system backup time. However, the UPS manual is vague about this. I hope this article can help you understand how to calculate UPS backup time and help you with your daily use.

Reference Video

1.Main UPS Categories

1.1 There are two types of UPS on the market

There are two types of UPS on the market, and their UPS backup time is calculated differently, so please make a distinction.

The UPS shown in the figure above has the battery built into the UPS as a device. The characteristic of this type of UPS is that the battery capacity is fixed. Therefore, when we need to calculate the backup time of this type of UPS, the first thing we need to know is the size of its built-in battery.

The UPS shown in the above picture does not have a built-in battery. Its battery is connected externally. This type of UPS provides better scalability. Users can increase the UPS backup time by increasing the battery capacity according to their own needs. However, to calculate the backup time of this type of UPS, it is necessary to confirm the capacity of its external battery.

2.Embedded Battery Type UPS Backup Time Calculation

2.1 Embedded Battery Type UPS Example

Source: APC UPS

The APC UPS in the picture above is a typical UPS with built-in batteries. Its battery module is fixed. You can usually find information about the UPS’s built-in batteries in the UPS’s product manual. Take this APC UPS as an example. Its specifications state the situation of its built-in batteries:

In fact, this UPS already has runtime information (i.e. UPS backup time). However, not all UPS manufacturers will provide this complete data. Therefore, we still calculate it based on its battery. You can see that after clicking on replacement battery, you can see the battery capacity it uses.

We have found out that this battery is a 12V7AH battery, and this parameter will be used in subsequent calculations.

2.2 UPS Backup Time Calculation

UPS backup time is related to battery capacity and load power.

We can think of the battery capacity of UPS as a water tank. There is a valve that allows water to flow out of the water tank, and the speed at which the water flows out of the water tank is the load power. So when the water flows out of the water tank is related to how much water is in the water tank and the water outflow speed. If we understand it from the perspective of UPS, UPS backup time is related to both the capacity of the UPS built-in battery and the load power.

Assume we have a 50W load and we already know that the battery capacity is 12V7AH. Then we can start calculating the UPS backup time.

First, the amount of power in the battery needs to be calculated. The amount of power in the battery is:

Electricity Storage in Battery= Battery Voltage*Battery Amps Hour = 12V*7AH = 84Watts-hour(Wh)

UPS Backup Time for 50W Load = Electricity Storage in Battery/Load Power = 84Wh/50W = 1.68hours

Please note that many times when the UPS is designed, the battery will not be deeply discharged completely. In many cases, the battery will stop discharging when there is 20% capacity left, thereby extending the battery life. So strictly speaking, we need to consider this factor.

UPS Backup Time for 50W Load with 20% Margin= UPS Backup Time for 50W Load *80% = 1.344hours

Please note that the above theoretical calculation can help you get a rough idea of UPS backup time, but the actual precise calculation is much more complicated. However, based on this, it can help you solve most problems in general projects.