In the electrical world, you’ll hear the terms “kVA” and “kW” a lot. Some folks use them like they mean the same thing—but they don’t. Mix them up, and you could end up with an overloaded transformer. Or a power system that doesn’t perform as expected. 

To understand the difference, you need to know what the three terms below mean.

What does the kVA rating mean?

kVA or kilovolt-amperes, is a unit of apparent power—the total power a transformer can supply. It represents the total "demand" placed on the transformer regardless of how much is converted into useful work.

You’ll see this rating on every transformer nameplate. It tells you two key things: the size of the transformer, and the maximum load the transformer is designed to handle. kVA is also used as a rating system for UPS (Uninterruptible Power Supplies).

What does the kW rating mean?

kW (kilowatts) measures real power. Real power is the actual power consumed by electrical equipment to perform useful work—such as lighting, heating, or running motors. kW tells us how much power is converted into mechanical work or heat. For example, a 22 kW motor actually delivers 22 kW of usable power to turn a shaft.

Why aren’t these terms interchangeable?

The reason these values aren’t identical is due to a lack of efficiency. In a 100% efficient system kW = kVA.

But most systems aren’t perfectly efficient. There’s some power lost in the process.

The relationship between kVA and kW depends on the power factor (PF) of your system. 

What is the Power Factor?

Power factor is the measure of efficiency in an electrical system. It is the ratio of usable/real power (kW) to total supplied/apparent power (kVA). 

Power Factor  = kW / kVA

This electrical efficiency is expressed as a power factor between 0 and 1. The closer the power factor is to 1, the more efficiently the kVA is being converted into useful kW. For most electrical systems, the power factor falls between 0.7 and 1.0, with 1.0 essentially being no power factor, or “unity”. A system at unity means all of apparent power is converted into real power.

Some power systems have no power factor at all. Systems with resistive loads, as well as no inductive or capacitive components, fall into this category.

The kW is always less than or equal to kVA. If you size your system assuming the two are the same, you could end up getting a transformer too small to power your equipment.

For example, if a transformer is rated at 100 kVA and the system operates at a PF of 0.8, the maximum usable output in terms of real power (kW) is:

100 kVA × 0.84 = 84 kW

Like kW, you can usually find the power factor of a motor on the nameplate.

Why do transformers use the kVA rating?

Transformers are rated in kVA because they must handle the total load demand, regardless of how efficiently that power is used. A transformer doesn’t know the details of its load. It only delivers voltage and current, so its size is based on apparent power (kVA), not the real power drawn by the load.

How do you convert between kW and kVA?

You can convert between these two values using a simple math equation: 

kVA = kW / PF

Remember, kVA refers to total supplied power, kW refers to usable power, and PF refers to the power factor. 

Here’s a quick example.

If you have a 400 kW load and a power factor of 0.8:

kVA = 400 kW / 0.8 PF = 500 kVA. 

So you’ll need a transformer with a rating of at least 500 kVA to support that load. 

If you’d like an easy way to calculate these numbers, use our kW to kVA calculator. 

Conclusion

kVA and kW sound similar, but they mean different things for your transformer. kVA refers to the total power capacity. kW refers to the usable power.

Always make sure your kW demand stays within your transformer’s kVA rating. Exceeding that limit will shorten your transformer’s life, hurt its overall performance, and lead to unexpected unit downtime. 

If you’re not sure which size transformer you need, fill out the form below. We’re here to get you the right unit for your project.