Generator Temperature Derating: Hot Weather Power Loss

What temperature derating means on a listing

When a generator listing shows a watt figure, that figure is the output the maker states under a specific set of conditions. One of those conditions is the temperature of the air the generator pulls in for cooling and combustion. Temperature derating is the term for the lower output the unit can deliver when the ambient air is hotter than the maker's rating condition.

A listing title rarely says "derating" or gives a temperature. The title's first watt figure is usually the starting watts, and the second figure on the maker's page is the running watts. Neither figure tells you what the unit will do at 100 degrees Fahrenheit. The maker's manual or spec sheet is where the rating temperature and any derating table appear. Read the listing for the watt figures, then open the maker's page to see whether it states a temperature for those figures.

Derating is not a defect. It is a normal consequence of how an air-cooled engine and an alternator work. The unit relies on moving air to carry heat away from the engine block, the oil, and the alternator windings. On a hot day, the air is warmer and carries less heat away per cubic foot, so the internal parts run hotter. To stay within the maker's design limits, the unit's available output is lower.

Why hot air reduces generator output

A generator makes electricity by burning fuel in an engine that turns an alternator. Both the engine and the alternator produce heat as they work. The cooling system, usually a fan that pulls air across the engine and the alternator, uses the surrounding air as its cooling medium. When that air is hot, the temperature difference between the hot metal parts and the air is smaller, so heat moves out of the unit more slowly.

The engine also breathes in the same hot air for combustion. Hot air is less dense than cool air, so each cylinder full contains less oxygen. With less oxygen, the engine cannot burn as much fuel to produce the same power. The combination of less dense combustion air and slower cooling is why the maker's rated output is only available up to a certain ambient temperature.

This is why a generator that starts and runs fine on a mild spring day may struggle to carry the same load during a summer heat wave. The unit is not broken. It is operating at a lower capacity because the environment gives it less cooling and less oxygen. The effect is larger on a fully loaded unit than on one running at a light load, because a loaded unit generates more heat.

How to find derating information for a specific model

The listing itself will not give you a derating curve. The maker's manual and the spec sheet are the places to look. Search the maker's site for the model number, open the manual, and look for a section on "ratings, output, or operating conditions. Some makers state the temperature at which the rated watts apply, such as 77 degrees Fahrenheit or 104 degrees Fahrenheit. Others provide a table that shows output at several temperatures.

If the maker's page does not state a rating temperature, treat the listed watts as the output at the maker's chosen condition, not as a promise for your climate. Ask the maker directly what the unit delivers at your local summer high. The maker's customer service line or the manual's contact page is the right place for that question.

The site's guides compare generators on stated details only. The guides do not measure output, so they cannot tell you how a specific model derates. The guides can help you narrow the field by watt group, fuel type, and features, but the derating question belongs to the maker. See the best generator guide for an overview of how the site compares units.

Sizing a generator for a hot climate

For a home in a region with regular summer highs above 90 degrees Fahrenheit, the practical approach is to choose a generator with more capacity than the loads require. The gap between the loads and the unit's rated watts is the margin that covers the derating on the hottest days. A unit that is rated just above the loads at 77 degrees may not carry them at 105 degrees.

The first step is to list the appliances and tools the generator will power, and note the starting watts and running watts for each. The starting watts are the extra power a motor needs for a moment when it starts. The running watts are what the item draws while it runs. Add the running watts of everything that runs at the same time, then add the largest starting watts figure. That total is the load the generator must carry.

The next step is to compare that load to the watt groups in the guides. If the load sits near the top of a watt group, move up to the next group to leave room for derating. For example, a load of 4,500 running watts is near the top of the 3,000 to 5,000 watt group. In a hot climate, a unit in the 5,001 to 9,000 watt group gives more margin for the hottest days.

The maker's manual is the authority on how the unit behaves at high temperatures. The manual may also state a maximum ambient temperature for operation. If the manual gives a derating table, use it to confirm that the unit can carry the load at your local summer high. If the manual gives no table, ask the maker.

For a camping trip or an RV in a hot region, the same logic applies. A small inverter set rated for 2,000 watts may not deliver that figure at 100 degrees. Choose a unit with a higher watt group than the camper's loads, and check the maker's page for the rating temperature. The camping generator guide and the RV generator guide list units by watt group, so the comparison is straightforward.

What the guides can and cannot tell you

The buying guides on this site compare generators on stated details: type, fuel, watt group, electric start, RV ready outlets, and CO sensor. The guides can help you find a unit in the watt group that leaves room for derating, and they can help you compare fuel types and features. The guides do not measure output, runtime, fuel use, or noise, and they do not test how a unit behaves in heat.

The guides also do not size a home. Sizing is a question for the maker and a licensed electrician, especially for a standby unit that connects to the home's wiring. For a portable unit, the maker's manual and the appliance nameplates are the sources for load figures. The guides are a starting point for the shopping list, not the final word on capacity.

For a home standby unit, the installation is done by a licensed electrician and the maker's authorized dealer. The dealer can size the unit for the home's loads and for the local climate. The whole house generator guide explains what standby means on a listing and what questions to ask an installer.

Related terms to read on a listing

Temperature derating is one of several conditions that affect a generator's available output. Altitude is another. At higher elevations, the air is thinner, so the engine gets even less oxygen per cylinder full. The maker's manual may state a derating for altitude as well as for temperature. The article on altitude adjustment explains that term.

The watt figures on a listing are the starting watts and the running watts. The starting figure is the larger one, and it applies for a few seconds when a motor starts. The running figure is the continuous capacity. Both figures are stated by the maker at a specific condition, so read them as the maker's claim, not as a guarantee for your site.

The fuel type also matters. A dual fuel or tri fuel unit may state different watt figures for gasoline and for propane or natural gas. The maker's page is where those per-fuel figures appear. The fuel comparison article explains how to read those figures.

Finally, the CO sensor and shutoff feature is a listing term that describes an automatic shutdown when the unit detects carbon monoxide. The feature does not change the unit's capacity. The CO sensor and shutoff is explained in the maker's manual and the CPSC and CDC pages linked in the sources.

What to pick for your situation

Questions

What is temperature derating on a generator?

Temperature derating is the reduction in a generator's available output when the ambient air is hotter than the maker's rating condition. The engine and alternator are cooled by the surrounding air, so on a hot day the unit cannot shed heat as quickly and delivers less power.

Does a generator listing show the derating figure?

Usually not. The listing title shows the starting watts and the maker's page shows the running watts, but the temperature at which those figures apply is in the maker's manual or spec sheet. Look for a ratings or operating conditions section, or ask the maker.

How much power does a generator lose in hot weather?

The amount varies by model and by the temperature. The maker's manual or spec sheet is the only source for a specific derating figure. The site does not measure output, so the guides cannot state a percentage.

How do I size a generator for a hot climate?

List the running watts of everything that runs at the same time, add the largest starting watts figure, and choose a generator in a higher watt group than that total. The margin covers the derating on the hottest days. Confirm with the maker's manual.

Does altitude affect generator output too?

Yes. At higher elevations, the air is thinner, so the engine gets less oxygen per cylinder full. The maker's manual may state a derating for altitude as well as for temperature. The article on altitude adjustment explains the term.

Can I use a generator indoors in hot weather to avoid derating?

No. Never use a portable generator indoors, in a garage, basement, crawlspace, shed, or near a window or vent. Carbon monoxide from a generator is deadly and can kill you in minutes. Use the generator outside only, far from the home, and follow the maker's manual and the CPSC and CDC guidance.