kW vs kVA Generator: What’s the Difference?
When you request a quote for a Cummins generator set, the spec sheet will show two power ratings side by side — kW and kVA. Most buyers assume they mean roughly the same thing, or simply focus on whichever number looks larger. In practice, confusing the two is one of the most common reasons a generator ends up undersized for the actual load, or oversized and burning more fuel than necessary.
This article breaks down what each rating means, how they relate to each other, and what to look at when you’re sizing a Cummins generator set for an industrial or commercial application.
What Is kW in a Generator?
kW stands for kilowatt, and it measures real power — the actual electrical output that does useful work. When a motor runs, a light turns on, or a heating element operates, the energy consumed is measured in kW. It’s the number that tells you how much of the generator’s output is being converted into productive work.
For most buyers, kW is the more intuitive figure. If your facility needs 150 kW of power to run all connected equipment, you need a generator that can deliver at least 150 kW of real power output.
What Is kVA in a Generator?
kVA stands for kilovolt-ampere, and it measures apparent power — the total electrical load the generator is carrying, which includes both real power (kW) and reactive power.
Reactive power doesn’t perform useful work in the traditional sense, but it’s not wasted either. It maintains the electromagnetic fields that inductive equipment including motors, transformers, compressors depends on to function. Without reactive power, these devices simply wouldn’t operate.
This is why generators are rated in kVA rather than kW alone. A generator has to supply both types of power simultaneously, so the kVA rating reflects the full capacity it needs to handle.
What Is Power Factor?
Power factor (PF) is the ratio between kW and kVA. It’s expressed as a number between 0 and 1, and it tells you how efficiently the apparent power is being converted into real, usable output.
The formula is straightforward:
kW = kVA × Power Factor
The standard power factor for industrial generators is 0.8. This is the value Cummins and most generator manufacturers use as the baseline for rating their equipment, because it reflects typical real-world load conditions, a mix of motors, lighting, HVAC, and other inductive equipment.
A power factor of 0.8 means that 80% of the apparent power (kVA) is doing real work (kW), while the remaining 20% is maintaining system stability. The closer the power factor is to 1.0, the more efficiently the generator is being used. Purely resistive loads like heating elements run at or near 1.0. Inductive loads like electric motors typically run between 0.7 and 0.85.
How to Read kW and kVA on a Cummins Generator Spec Sheet
On a Cummins generator set datasheet, you’ll see both kW and kVA listed — along with the power factor and the operating mode. Here’s what a typical entry looks like:
Prime Power: 100 kW / 125 kVA @ 0.8 PF Standby Power: 110 kW / 137.5 kVA @ 0.8 PF
Two things to pay attention to here.
First, prime power vs. standby power is not the same rating. Prime power is the continuous output the generator can sustain for unlimited hours with a variable load. Standby power is the maximum output available during a grid outage — it’s higher, but it’s only intended for limited hours per year and cannot be run continuously. Using a standby-rated generator as a primary power source will shorten its service life considerably.
Second, notice how the kVA number is always higher than kW at 0.8 PF. A generator rated at 100 kW and 125 kVA isn’t two different machines, it’s the same machine described from two different angles. The kW tells you the real output; the kVA tells you the total capacity required to deliver it.
You can explore Longshine’s Cummins generator set range, including standard, silent, and rainproof configurations, each with full kW and kVA specifications.
How to Convert Between kW and kVA
Once you know the power factor, converting between the two is a simple calculation.
To convert kW to kVA: kVA = kW ÷ Power Factor
Example: Your total connected load is 200 kW, and your load type gives you an estimated PF of 0.8. 200 ÷ 0.8 = 250 kVA — that’s the minimum generator rating you need.
To convert kVA to kW: kW = kVA × Power Factor
Example: A generator is rated at 250 kVA at 0.8 PF. 250 × 0.8 = 200 kW of real power output.
If you’re unsure about your load’s power factor, 0.8 is a safe and widely accepted default for mixed industrial loads. For facilities with a high proportion of motors or compressors, you may want to verify with an electrical engineer, as the actual PF could be lower.
Common Generator Sizing Mistakes to Avoid
Looking only at kW and ignoring kVA. If your load includes motors, pumps, or compressors, the generator needs enough kVA capacity to handle both the real and reactive demand. A generator selected purely on kW may trip under load because it can’t handle the full apparent power requirement.
Using the standby rating for continuous operation. This comes up often when buyers compare specifications across manufacturers. A 110 kW standby rating sounds better than a 100 kW prime rating — but they’re not equivalent. For continuous or primary power applications, always size against the prime power rating.
Underestimating motor starting loads. When an electric motor starts up, it draws three to seven times its normal running current for a brief period. If the generator is sized too tightly, that starting surge can cause voltage dips or even shut the unit down. According to Cummins Power Generation guidelines, motor starting capacity should be factored into the sizing process separately from steady-state load.
Ignoring future load growth. A generator sized exactly for today’s load leaves no room for expansion. If additional equipment comes online later, you may end up running the generator at or above its rated capacity — which accelerates wear and increases the risk of failure.
So Which Rating Should You Use When Buying a Generator?
Both matter, but here’s a practical sequence to follow:
- Add up your total connected load in kW. Include all equipment that will run simultaneously, not just the largest single item.
- Identify your load type. Mostly resistive (lighting, heating)? Your PF is closer to 1.0. Mostly inductive (motors, pumps, compressors)? Plan on 0.8 or lower.
- Calculate the required kVA using: kW ÷ PF = kVA.
- Add a buffer of 15–20% to account for motor starting surges, load variations, and future expansion.
- Check whether you need prime or standby power and match the rating accordingly.
For example: a food processing facility with 160 kW of connected load, predominantly motors, at PF 0.8: 160 ÷ 0.8 = 200 kVA → add 20% buffer → minimum 240 kVA generator.
That’s the kind of calculation that prevents an undersized unit from causing operational problems six months after installation.
If you’re comparing Cummins generator set models or need help matching a unit to a specific application, the Longshine team can assist with selection. Browse the Cummins generator set product range or get in touch with your load requirements.