A generator's rating is stated at reference conditions. Real installations are frequently not at those conditions, and the difference matters when the sizing is close.
Altitude
Air is less dense at elevation, so an engine draws less oxygen per stroke and produces less power.
Manufacturers publish the derating, and it is typically a few percent per thousand feet above a threshold. At a mile of elevation the reduction is substantial — enough to move a generator down a size in effective output.
Naturally aspirated engines are affected more than turbocharged ones, and almost all residential air-cooled units are naturally aspirated.
Ambient temperature
Above a reference temperature, output falls again, for the same density reason plus the reduced ability to reject heat.
That matters because the summer peak load — air conditioning — coincides exactly with the highest ambient temperatures. A generator sized for a 3-ton compressor at 20°C may not start it at 40°C.
Cold does not help as much as it seems
Cold, dense air is good for the engine, and cold weather brings its own problems: a battery at reduced capacity, thicker oil, and on propane a reduced vaporisation rate from the tank.
Cold-weather kits — a battery warmer, an oil heater — exist for this and are standard practice in severe climates.
Where derating changes a decision
A house at elevation with a marginal generator selection. Two thousand feet is enough to matter and five thousand is a different generator.
A hot climate, where the summer condition is the design condition.
A tight sizing where a soft starter and shedding have already been used to make a smaller unit work. Derating eats the remaining margin.
How to apply it
Take the calculated load, then take the generator's derated output at the site's elevation and expected summer ambient, from the manufacturer's tables.
Compare those two rather than comparing the load to the nameplate. A generator whose nameplate exceeds the load by 15% may be under it once derated.
What the installer should provide
The derated output figure for the site, alongside the load list.
A quote that names a kW rating and a house is not enough on its own where either elevation or summer temperature is significant, and the tables are in the same document the installer used to select the unit.
Fuel makes a small difference too
The same engine produces slightly less on natural gas than on propane, typically a few percent, because of the energy content per unit volume delivered.
Where a generator is field-convertible, the rating differs between the two fuels and both are published. On a marginal selection, this is one more few-percent item that can add up with the others.
The margin worth keeping
After derating, after the largest starting surge, and after the shedding arrangement, it is worth leaving some headroom rather than sizing to exactly the calculated peak.
Loads get added over the years — a freezer in the garage, a heat pump water heater, a vehicle — and a generator with no margin is one that becomes inadequate quietly.
The same applies to portable units
Portable generators are derated for elevation in exactly the same way, and their manuals say so.
Households taking a portable unit to a mountain cabin frequently find it will not start a well pump it handled at home, and the reason is entirely this.
Some manufacturers supply a high-altitude jet kit, which restores correct combustion at elevation and does not restore the lost output — it prevents the engine running rich, which is a durability question rather than a power one.
Where the numbers come from
Manufacturers derate against a published standard reference condition, and the tables appear in the specification sheet rather than the marketing material.
Ask for the spec sheet, not the brochure. It contains the derating curves, the fuel consumption at 50% and 100% load, the sound level, and the extended electrical data — which together are most of what any sizing decision needs.