“Why Does the Same Generator Make Less Power on Natural Gas — and Does It Change Which One I Buy?”
Almost every shopper trips over the same line on the spec sheet: one number for liquid propane, a lower one for natural gas. People assume it’s a marketing asterisk or a defect. It is neither — it’s combustion physics, and it shows up on every gaseous standby including both the Generac Guardian and the Kohler 26RCAL. The reason it’s worth understanding rather than memorizing is that once you know why the derate happens, you also know exactly when it changes the brand decision and when it doesn’t. So we build the answer from the mechanism up, then let it resolve the buy.
Why is the natural-gas rating lower than the propane rating on the same machine, and should that derate steer me toward one brand?
An engine makes power from energy per intake stroke, and NG delivers less of it
A naturally aspirated engine ingests a roughly fixed volume of air-plus-fuel each cycle. Power out depends on the chemical energy in that volume. Liquid propane vapor carries more energy per unit volume in the mixture than natural gas (mostly methane) does at the carburetor. Same displacement, same RPM — but less energy per stroke on NG, so less power at the crank. Nothing is broken; the fuel simply brings less to the table.
This is why the derate is universal and predictable, not a brand quirk. Fuel burn, separately, tracks load times the engine’s specific fuel consumption — heavier load draws proportionally more gas — but the ceiling on usable power is set by which fuel feeds the carburetor.
On NG, the badges you were comparing aren’t the numbers you get
| Model | LP rating | NG rating (what you size to) |
|---|---|---|
| Generac Guardian 24 kW (7210) | 24 kW | 21 kW |
| Kohler 26RCAL | 26 kW | 24 kW |
Read on the badge, these look one tier apart (24 vs 26). Read on natural gas — the fuel most suburban homes actually have — the usable gap is 21 vs 24 kW, a wider 3 kW spread. The mechanism from Stage 1 is what opens that spread: the same physics applied to two machines with different displacements and tuning lands them at different NG ceilings.
The derate sets the ceiling; load management decides whether you hit it
Motor-start sizing is governed by locked-rotor amps versus surge headroom, and the worst case is several big motors inrushing together. Both machines stagger those starts — Generac generator with Smart Management Modules, Kohler generator with the RXT switch’s built-in load-management board and current transformer — so the peak simultaneous demand never forms. That lowers the kW you must actually clear, which interacts directly with the NG derate.
The answer, as a rule
The NG derate is combustion physics — natural gas brings less energy per intake stroke than propane, so every gaseous standby makes less power on it. It is universal, predictable, and not a brand defect. Whether it steers your purchase comes down to one comparison: your natural-gas load (quoted with load management active) against each unit’s NG ceiling — 21 kW for the 24 kW Guardian, 24 kW for the Kohler 26RCAL. The numeric lines: if your staggered NG load is under ~21 kW, both clear and you decide on price, controller, and dealer; if it lands between ~21 and ~24 kW, the derate selects the Kohler at its badge or sends the Guardian one model up; if it exceeds ~24 kW on NG, neither of these two clears and you step up the range regardless of brand. Size to the derated, load-managed number — never the propane badge.
Topology/standards per the cited standards; all product ratings are manufacturer-stated values from the cited datasheets, current to 2026-06; derived/illustrative figures are labelled as such. This is not an independent head-to-head test. Generac is a brand affiliated with this site; competitor names are used for identification only.
