Altitude + heat derate
Updated October 6, 2026.
Built by Jeremy Panasuk.
Use this after you know the watts you need and before you trust a nameplate at elevation or in hot inlet air. The button applies this site’s curve. Generac’s and Honda’s sentences are next to it, and they are not the same curve.
The curve in the button
GS.derateFactor in assets/lab.js:
- Altitude term = 0 at or below 1,000 ft. Above 1,000 ft it is 0.035 × (elevation − 1,000) / 1,000.
- Heat term = 0 at or below 77°F. Above 77°F it is 0.01 × (temperature − 77) / 10.
- Factor = 1 − altitude term − heat term, and never below 0.55.
- Usable watts = nameplate × factor. The page rounds that with
Math.round.
The 1,000 ft exemption, the 77°F start of the heat term, and the 0.55 floor are this script. They are not copied from the manuals below.
Generac’s GP6500/GP8000E owner’s manual says maximum power decreases about 3.5% for each 1,000 feet above sea level, and about 1% for each 10°F above 60°F. The same manual says that when the generator is operated above 77°F there may be a decrease in power. That is a separate sentence from the 1%-per-10°F line, which Generac counts from 60°F. The 3.5% step is the part this tool shares. The tool starts that step after 1,000 ft, not at sea level, and it starts its 1% step at 77°F, not at 60°F.
The Honda EU2200i owner’s manual says that even with the high-altitude carburetor modification, engine horsepower decreases about 3.5% for each 1,000-foot increase in altitude, and that the effect is greater if the modification is not made. Honda tells you to have a dealer do that modification if you always operate above 5,000 feet. That manual’s altitude section does not give a percent per 10°F.
Worked clicks
The form opens at 7,500 W nameplate, 1,050 ft, 90°F, and 4,200 W needed. 7,500 and 4,200 are typed defaults, not a model’s rated watts. Generac’s manual lists 6.5 kW rated for the GP6500 and 8.0 kW rated for the GP8000E.
- Altitude term = 0.035 × (1,050 − 1,000) / 1,000 = 0.00175.
- Heat term = 0.01 × (90 − 77) / 10 = 0.013.
- Factor = 1 − 0.00175 − 0.013 = 0.98525, shown as 98.5%.
- Usable = 7,500 × 0.98525 = 7,389.375, shown as 7,389 W. Against 4,200 W needed, the button says it covers, by 3,189 W.
Second click: 7,500 W, 5,000 ft, 60°F, need still 4,200 W. Five thousand feet is Honda’s carburetor-modification threshold, not a claim about a town. At 60°F the tool’s heat term is 0, because 60 is below 77.
- Altitude term = 0.035 × (5,000 − 1,000) / 1,000 = 0.14. Factor = 0.86, shown as 86.0%. Usable = 6,450 W, which still covers 4,200 by 2,250 W.
- Generac’s sentence counted from sea level, by hand, is 3.5% × 5 = 17.5% off, leaving 82.5% and 6,187.5 W. The button does not print 6,187. It skipped the first 1,000 ft.
How to read the derate table
Read the percent, the starting elevation, and the starting temperature as three separate sentences. Generac's GP6500/GP8000E manual starts the 3.5% step at sea level and the 1% per 10°F step at 60°F. A separate sentence says power may drop when the generator is operated above 77°F. That sentence does not replace the 60°F line. The Honda EU2200i manual says engine horsepower decreases about 3.5% for each 1,000-foot increase in altitude even with the high-altitude carburetor modification, and it tells you to have a dealer do that modification if you always operate above 5,000 feet. Honda's altitude section does not give a percent per 10°F. Do not borrow Generac's temperature line and call it Honda's.
Third click on this page's curve, which is not Generac's sea-level sentence: 7,500 W nameplate, 2,000 ft, 100°F, need 4,200 W. Altitude term = 0.035 × (2,000 − 1,000) / 1,000 = 0.035. Heat term = 0.01 × (100 − 77) / 10 = 0.023. Factor = 1 − 0.035 − 0.023 = 0.942, shown as 94.2%. Usable = 7,500 × 0.942 = 7,065 W, which still covers 4,200. Generac's sentence counted from sea level and from 60°F, by hand, is 3.5% × 2 = 7% for altitude plus 1% × (100 − 60) / 10 = 4% for temperature, 11% off, leaving 7,500 × 0.89 = 6,675 W. The button does not print 6,675.
Limits and a common mistake
The 0.55 floor binds when the uncapped factor would be lower. At 14,000 ft and 60°F the altitude term is 0.035 × 13 = 0.455, so 1 − 0.455 = 0.545, and the script returns 0.55. Usable watts on a 7,500 W nameplate are 4,125. Generac’s sea-level sentence with no floor, applied by hand to 14,000 ft, is 3.5% × 14 = 49% off, leaving 3,825 W. The button will not show 3,825.
A dealer sheet that says full output to some elevation replaces this curve for that machine. A turbocharged standby is that case. Sequence the load on the load sequencer before you type “watts you need,” and check lot-line noise if the fix for a short derate is a bigger open frame. This page does not pick a transfer switch. Permanent installation takes a licensed electrician and a permit where local code requires one.
Compiled with AI assistance from the sources below. Spot an error? Email [email protected].