Altitude + heat derate
Last updated: September 28, 2026 (CT). A 7,500 W carton in thin air is a smaller engine. This page is the correction.
Generic sizers ignore elevation. We apply the same family of rules printed on Kohler / Generac / Cummins portable sheets: no altitude penalty to 1,000 ft, then about 3.5% per additional 1,000 ft, plus about 1% per 10°F of inlet air above 77°F. The factor never drops below 55% in this tool so a 10,000 ft shop still gets a usable figure instead of a blank.
Worked check — New Auburn vs a mountain cabin
New Auburn, Wisconsin sits near 1,050 ft. At 90°F the factor is about 98.7% — a 7,500 W unit still has ~7,400 W. Leadville, Colorado (~10,150 ft) at the same temperature is a different machine: roughly 3.5% × 9.15 extra thousand feet ≈ 32% altitude hit, plus ~1.3% heat, factor ≈ 0.67. That 7,500 W sticker becomes about 5,000 W usable. If the sequencer said you need 5,200 W, you do not “have headroom.” You buy the next class or you shed the wait lane.
Turbocharged standby plants follow the manufacturer’s own curve, not this portable rule. If a dealer quotes “full power to 5,000 ft,” use their sheet and ignore this page for that unit.
How to use it with the other lab tools
Sequence first (load sequencer), paste that buy-class into “watts you need,” then derate the unit you were about to click on Amazon. If usable < need, the listing is the wrong elevation for your pad. Then check lot-line noise — a bigger open-frame to recover derate is also a louder fence.