Read the exact model and rating
Before comparing capacity, record the complete generator model and configuration. Check the current specification sheet for fuel, phase, voltage, frequency, standby or other rating, and any conditions printed in footnotes. A product family can include multiple outputs or electrical configurations; the headline kW alone does not identify the unit being proposed.
As one example, Cummins lists the exact RS20A at 20 kW / 18 kVA standby on its model page. This is the manufacturer’s rating for that model—not a generic conversion rule for other generators.
Separate the fuel-specific output
Some dual-fuel standby generators publish different output on natural gas and LP. For example, Generac’s G0073270 is listed at 26 kW on LP and 24 kW on natural gas; the single-phase KOHLER 26RCA is also listed at 26 kW LP and 24 kW natural gas. Do not compare one model’s LP rating with another model’s natural-gas rating and call them equal.
Verify that the proposed fuel supply can meet the selected model’s inlet requirements while the planned loads are operating. See the natural-gas versus propane guide for exact-model examples and supply questions.
Altitude, temperature, and site limits
Manufacturers may publish environmental derating guidance for an exact model or family. Generac’s cited 22–28 kW sheet states that maximum output decreases with altitude and temperature and provides approximate model-family factors. Those factors are not universal, and exact available capacity also depends on fuel, ambient conditions, humidity, barometric pressure, fuel quality, and equipment condition.
Give the installer the property elevation and expected ambient conditions. Ask them to show how the exact manual/specification sheet and site conditions affect the design; do not treat the brochure’s base rating as guaranteed output at every location.
Running load and motor starting are different questions
Motors and compressors can draw more current while starting than after they are running. The amount depends on the exact equipment, controls, starting method, and what else is operating. Use the equipment nameplate, technical manual, or measured/design data—not a generic “times running watts” multiplier.
In a written proposal, ask which equipment may start while other loads are already running, how load management affects that sequence, and which fuel-specific generator rating applies. The appliance wattage chart is only a generic manufacturer-published reference; it cannot replace the exact nameplate.
What the professional design should resolve
- Exact connected loads and which ones are expected to operate at the same time.
- Motor-start data, load-management controls, and any restrictions on simultaneous operation.
- Generator output on the actual fuel, with applicable site derating.
- Transfer-switch configuration, voltage/phase, utility service, and approved installation method.
- Fuel-system capacity, permits, inspections, local code, and manufacturer installation requirements.
Standby Generator Guide has not performed an on-site load calculation. Use this article to prepare questions, then have a qualified local professional produce and explain the property-specific design.