Some generator and appliance specs are listed in kW. Others, especially larger standby units and anything with a motor-heavy load, are listed in kVA. Treating the two as interchangeable is a quiet way to undersize a generator without ever realizing the math was off.
What the Difference Actually Is
Watts (kW) measure real power, the power that actually does useful work, whether that's spinning a motor or lighting a bulb. Volt-amps (kVA) measure apparent power, the total power the electrical system has to supply, including a portion that doesn't do useful work but still has to flow through the wiring.
The ratio between the two is called power factor, a number between 0 and 1 that describes how efficiently a load converts apparent power into real power. Purely resistive loads, like an incandescent bulb or a space heater, have a power factor close to 1, meaning kW and kVA are nearly identical for them. Motors and anything with an inductive coil, refrigerator compressors, well pumps, air conditioners, have a lower power factor, often somewhere around 0.8, meaning their kVA draw is meaningfully higher than their kW draw.
Why This Matters for Generator Sizing
Generators are often rated in kVA precisely because that's the number that describes what the generator's alternator actually has to be capable of supplying, regardless of what kind of load is attached. If you take a generator's kVA rating and assume it equals kW, you're overestimating how much real running-watt capacity you actually have.
A generator rated at 3 kVA with a typical 0.8 power factor assumption is really only reliably delivering about 2.4 kW to a motor-heavy load. If your appliance list was built entirely in kW and you compare it directly against the generator's kVA number without converting, you'll believe you have more headroom than you actually do, right up until a motor-driven appliance starts and the math doesn't hold.
How to Convert Between Them
The formula is straightforward once you know it: kW = kVA times power factor. If a spec sheet doesn't list power factor directly, 0.8 is a reasonable conservative assumption for a mixed household load with refrigeration and pump motors involved. For a load that's mostly lighting and electronics, 0.95 or higher is closer to reality, since those loads have power factors close to 1.
The direction of the conversion matters. Going from kVA to kW, multiply by power factor, which gives you a smaller number. Going from kW to kVA, divide by power factor, which gives you a larger number. Mixing up the direction produces an answer that's wrong in the optimistic direction, which is the worse way to be wrong when you're sizing backup power.
A Worked Example
Say you're comparing two generators for a household with a well pump, a refrigerator, and general lighting. One listing shows 3,500 watts. The other shows 3.8 kVA. On the surface, the kVA unit looks bigger.
Applying a 0.8 power factor to the kVA-rated unit, since the load is motor-heavy, gives 3.8 times 0.8, or roughly 3,040 real watts, actually less than the first unit's 3,500 watt rating despite the larger-looking number on the label. Buying the "bigger" kVA-rated unit without converting would have meant buying less real capacity than the alternative, exactly backwards from what the shopper intended.
This kind of mismatch shows up often enough in real listings, especially when comparing a portable unit rated in watts against a whole-house standby unit rated in kVA, that skipping the conversion step is a genuine way to end up with the wrong generator even after doing careful research. It's also an easy mistake to repeat across multiple purchases, since nothing about the buying experience itself flags that a conversion was needed in the first place.
Doing This Without a Spreadsheet
Manually tracking kW versus kVA across a mixed appliance list, on top of the running-versus-starting watt distinction that also matters for generator sizing, is exactly the kind of math that's easy to get subtly wrong under time pressure. The Electrical Power Calculator handles kW-to-kVA conversion alongside amp and volt calculations, so the power-factor step doesn't get skipped just because it's less familiar than watts.
For the full sizing walkthrough, including how starting load factors in on top of this, see How to Size a Backup Generator Before the Next Outage, published on the EvvyTools blog.
Why Power Factor Isn't Fixed
Power factor isn't a single number that applies to every motor equally, which is part of why the 0.8 assumption is a starting point rather than a guarantee. Older motors and cheaper appliances tend to run at a lower power factor than newer, more efficient designs, since manufacturers have gotten better over time at reducing the reactive power component of a motor's draw. A modern high-efficiency refrigerator compressor might run closer to 0.9, while an older well pump motor might sit closer to 0.7.
This is one of the reasons two households with what looks like the same appliance list on paper can end up needing meaningfully different generator capacity. The appliances themselves, not just their wattage ratings, determine how much the kVA-to-kW gap actually matters for a specific sizing decision.
What This Looks Like on an Actual Spec Sheet
Whole-house standby generators almost always list their primary rating in kW, but their alternator's underlying capacity is often documented separately in kVA in the technical spec sheet, sometimes buried a page or two into the manual rather than on the main product listing. Portable units are more likely to lead with watts, but higher-end inverter models occasionally list both figures side by side specifically because installers and electricians expect to see kVA.
If you're only shopping by the headline number on a retail listing page, you may never see the kVA figure at all, which is exactly why it's worth pulling the full spec sheet or manual before comparing two units that use different units for their primary rating. A five minute detour into the manufacturer's technical documentation, rather than relying entirely on the retail listing's summary box, is often enough to catch a kW-versus-kVA mismatch before it becomes an expensive mistake.
This is particularly worth doing when comparing units across different sales channels, since a big box retailer's product page and a specialty electrical supplier's spec sheet for the exact same unit sometimes present the rating differently depending on which audience each channel expects to be reading it.
References on Power Factor and Apparent Power
NEMA publishes standards documentation covering motor power factor and electrical equipment ratings that generator and appliance manufacturers work from. Wikipedia's entry on power factor is a solid general explainer of the relationship between real, apparent, and reactive power. The U.S. Department of Energy has consumer material on generator specifications that touches on the kW-versus-kVA distinction in plain language.
Watts and volt-amps look similar enough on a spec sheet that it's easy to assume they're the same thing. For anything with a motor attached, they aren't, and the gap is exactly the kind of detail that turns "should be more than enough" into "why did it just trip." A single multiplication by power factor is all it takes to close that gap before it becomes a problem instead of after.
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