Buying a UPS does not mean that bigger is always better

You can think of a UPS like a large-engine car driving on a highway. When the car is operating at a suitable speed and load, the engine works within its optimal range, providing good performance while consuming fuel efficiently.

A UPS works in a similar way. For most UPS systems, the conversion efficiency is usually highest when the load is within a moderate range, often around 40%–70%, with efficiency potentially reaching 95%–97%, depending on the model and operating conditions.

However, if the load is extremely low—for example, below 10%—the efficiency of some UPS systems can decrease significantly.

Imagine using a large-engine car just to carry one case of bottled water around the city. Although the car has plenty of power, it may consume much more fuel than necessary.

The same principle applies to UPS systems.

Therefore, choosing a UPS is not about selecting the largest capacity possible, but about choosing the right capacity so that the UPS can operate comfortably and efficiently.

A professional UPS selection should consider the actual load, load type, starting current, future expansion requirements, and required backup time.

The right UPS capacity provides the best balance between reliability, efficiency, and cost.

So Buying a UPS does not mean that bigger is always better. The UPS capacity should be selected according to the actual load and the appropriate capacity margin.

  1. A UPS That Is Too Small: Definitely Not Recommended

For example, if the actual equipment load is 8kW, but you choose a 5kVA UPS, it may result in:

Long-term UPS overload

Frequent transfer to bypass mode

Reduced battery runtime

UPS protection shutdown in severe cases

2. A UPS That Is Too Large: Not Necessarily Cost-Effective

For example, if the actual load is only 3kW, but you purchase a:

20kVA / 20kW UPS

Although it can handle the load, it may result in:

Higher initial purchase cost

Potentially larger battery configuration

Larger equipment size

Efficiency/economics may not be optimal at very low load levels

Higher maintenance costs over the long term

Therefore, UPS sizing is not about:The bigger, the better.

It is about:Actual load + reasonable capacity margin = the right UPS capacity

3. How Do We Generally Calculate the UPS Capacity?

For example, if the customer's actual load is:8kW

We can consider reserving approximately 20%–30% capacity margin.

A simple calculation would be:

8kW × 1.25 = 10kW

Therefore:A 10kW / 10kVA UPS

would generally be a reasonable choice.

This is also why the 10kVA UPS with PF=1 for your previous Bangladesh customer would generally be suitable if the final load is around 8kW.

4. However, There Is Another Very Important Factor: Load Type

You cannot select a UPS based only on the number of kW.

For Servers / IT Equipment

You should mainly consider:

· kW

· kVA

· Power Factor

· Starting/inrush current

For CNC Machines, Laser Equipment, CT Scanners and Industrial Equipment

You also need to pay special attention to:

· Motors

· Compressors

· X-ray generators

· Variable-frequency drives (VFDs)

· Inductive loads

· Starting current

· Instantaneous peak power

Even if this type of equipment has a normal operating load of only 8kW, you should not automatically select a 10kVA UPS.

Depending on the actual starting current and load characteristics, the UPS capacity may need to be increased:

10kVA → 15kVA → 20kVA

The final selection should be based on the actual equipment parameters and load characteristics.

5. Another Common Misunderstanding

A higher-capacity UPS does NOT necessarily mean longer backup time.

For example:

10kVA UPS + 40 × 9Ah batteries and 20kVA UPS + 40 × 9Ah batteries

does not mean that the 20kVA UPS will provide longer backup time.

Backup time mainly depends on:

Battery capacity + actual load power + UPS efficiency

For example, with the same battery bank:

5kW load → longer backup time

10kW load → significantly shorter backup time

The right UPS is not necessarily the biggest UPS. It is the UPS that is properly sized for the actual load, load characteristics, future expansion, and required backup time.