Generator ratings come in several forms and they describe different duty cycles.
Standby rating
The output available for the duration of a utility outage, with a varying load, for a limited number of hours per year.
This is what residential standby generators are rated at, and it is the appropriate basis: a house generator runs a few dozen hours a year, occasionally for a few days.
Prime and continuous ratings
Prime power is for running as the main supply with a varying load, for unlimited hours.
Continuous is for a constant load, unlimited hours.
Both are lower than the standby rating for the same machine, because sustained operation is harder on it. They apply to construction sites, remote installations and telecoms, not to houses.
A generator sold with a prime rating is being described more conservatively, which is worth noting when comparing across manufacturers.
Surge and running watts
On portable generators the marketing figure is usually the surge or peak watts — the output available for a few seconds to start a motor.
The running or rated watts is the continuous figure, and it is typically ten to twenty percent lower.
Sizing against the surge number is the classic portable generator mistake: a unit sold as "8,000 watts" may produce 6,500 continuously, which is a different generator for a load list.
kW and kVA
kW is real power, what the loads consume.
kVA is apparent power, which is what the alternator has to supply.
For resistive loads they are the same. For motors they are not, and the ratio is the power factor, conventionally 0.8 for a generator's rating.
A generator rated 22 kW at 0.8 power factor is 27.5 kVA. Residential specifications generally quote kW, and the distinction matters mainly when comparing a residential unit against commercial equipment.
Natural gas and propane ratings
The same engine produces slightly less on natural gas than on propane, and both figures are published.
A unit sold as "22 kW" is often 22 on propane and slightly less on natural gas. On a marginal sizing that difference is worth checking.
Amps, not just kilowatts
At 240 V, kilowatts divided by 240 gives amps — a 22 kW generator is roughly 92 amps.
That is the number to compare against the transfer switch rating and against what the house draws, and it makes clear why a generator does not simply replace a 200 amp service without load management.
What to compare between quotes
The standby rating on the fuel that will actually be used.
The derated output at the site's elevation and expected summer temperature.
The output in amps, against the load list.
Two quotes naming the same headline kW can differ once those three are applied, and the specification sheet — not the brochure — is where all three live.
The margin question
A generator sized to exactly the calculated peak has nothing left for the freezer that gets added to the garage, or the vehicle bought two years later.
Some headroom is reasonable. A great deal of headroom is money spent on a machine that will spend its life at a low load factor, where engines are least efficient and where wet stacking becomes a concern on diesel units.
Ten to twenty percent above the derated requirement is the usual sensible landing point.
Air-cooled and liquid-cooled at the same rating
Two units of the same kW can be built very differently.
An air-cooled unit runs a smaller engine at a higher speed and is rated for intermittent standby duty. A liquid-cooled unit at the same output runs a larger engine more slowly, is quieter, and is built for far more hours.
At the top of the air-cooled range the two overlap in output and not in price or service life, and which is appropriate depends on how often the local grid actually fails.