Generator rated output does not fall at one universal rate as ambient temperature rises. The temperature at which derating begins—and the reduction above that temperature—depends on the generator model and configuration. For example, Generac’s published Protector diesel specification gives a reduction of 3% for every 5°C above 25°C, while its SD100 specification lists 43.3°C as the maximum ambient temperature before derating. These values are not interchangeable.
What This Table Answers
Start with the generator model, its applicable electrical power rating, and the ambient temperature specified by its manufacturer. The table below returns the temperature-adjusted output for a 30 kW Generac RD030 standby example, calculated from the manufacturer’s published temperature correction. It is a model-specific planning reference, not a derating chart for other generators. The source lists the RD030 at 30 kW standby and 24 kW prime; the calculations here use only the standby rating.
Generator Output vs Ambient Temperature Table
The Generac Protector diesel specification, pages 4–5, lists the RD030 standby output and a temperature reduction of 3% per 5°C above 25°C. Its revision/date could not be established from the accessible text; confirm the applicable document against the unit’s model and serial number before using these figures for equipment selection.
| Ambient temperature | Temperature reduction | Output remaining | Calculated RD030 standby output | Source and revision status |
|---|---|---|---|---|
| 25°C / 77°F | 0% | 100% | 30.0 kW | Protector specification, pp. 4–5; revision/date not established |
| 30°C / 86°F | 3% | 97% | 29.1 kW | Protector specification, pp. 4–5; revision/date not established |
| 35°C / 95°F | 6% | 94% | 28.2 kW | Protector specification, pp. 4–5; revision/date not established |
| 40°C / 104°F | 9% | 91% | 27.3 kW | Protector specification, pp. 4–5; revision/date not established |
| 45°C / 113°F | 12% | 88% | 26.4 kW | Protector specification, pp. 4–5; revision/date not established |
| 50°C / 122°F | 15% | 85% | 25.5 kW | Protector specification, pp. 4–5; revision/date not established |
The percentages and adjusted outputs are calculated from the published Celsius rule, not separately measured manufacturer performance points. The same specification lists 50°C as the maximum ambient temperature; the table does not authorize operation above that limit or establish performance under additional site restrictions.
How to Calculate the Temperature Adjustment
For the published Protector rule, the tabulated values use:
[
P_T=P_r\left[1-0.03\left(\frac{T-25}{5}\right)\right]
]
Where:
- (P_T) is temperature-adjusted electrical output, in kW.
- (P_r) is the applicable electrical rating before this temperature correction, in kW.
- (T) is ambient temperature, in °C.
- (0.03) represents a reduction of 3% of the original rating per 5°C above 25°C.
This expression applies the published reduction linearly to the original rating. Use it only where the model’s documentation supports that rule and operating range. Do not apply it below 25°C to claim additional output. The table uses Celsius consistently rather than mixing the manufacturer’s rounded Celsius and Fahrenheit correction rates.
Worked Example: 30 kW Standby at 40°C
Assume an RD030 with the documented 30 kW standby rating, an ambient temperature of 40°C, and no additional altitude or installation-related reduction.
The temperature is 15°C above the 25°C correction threshold:
[
\text{Temperature reduction}
=0.03\times\frac{40-25}{5}
=0.09
]
[
P_T=30\times(1-0.09)=27.3\text{ kW}
]
The result is a temperature-adjusted standby estimate of 27.3 kW. It is not a prime or continuous rating, and it does not establish that the generator can start a particular motor or accept a particular load step. The manufacturer lists motor-starting capability separately from steady electrical output.
Check the Temperature Limit, Not Just the Cooling Rating
A maximum operating temperature is not necessarily the temperature at which full rated output remains available.
For example, the Generac SD100 specification, Part No. 0K5093, Rev. C, dated June 8, 2015, lists:
- Standby output: 100 kW.
- Maximum ambient temperature before derating: 43.3°C / 110°F.
- Maximum operating radiator air temperature: 50°C / 122°F.
That document does not provide a numerical temperature-derating slope. It therefore does not support calculating the SD100’s output at 50°C by borrowing the Protector’s 3%-per-5°C rule. Obtain the applicable manufacturer derating data for the actual configuration; this dated specification should not be assumed to describe every current SD100 unit.
How to Use the Result for Site Planning
Before comparing adjusted output with a load list, check the following:
- Match the rating category. Use the documented standby, prime, or other applicable rating as the starting value; do not substitute one for another. The Protector specification lists different standby and prime outputs.
- Check altitude separately. The Protector document provides altitude corrections in addition to temperature corrections. Where both apply, obtain the manufacturer’s combined-condition method rather than assuming the reductions should be added or multiplied.
- Check the air reaching the equipment. The Protector installation drawings require fresh cooling air and prevention of radiator-discharge recirculation. A nearby weather reading alone does not establish the generator’s actual cooling conditions.
- Retain electrical and starting checks. Compare the load with the applicable nameplate voltage, phase, current and power-factor ratings, and assess motor starting or load-step performance separately. A temperature-adjusted kW figure does not replace those checks.
Use the table to estimate the temperature component of available output. Have the supplier or qualified installer confirm the actual unit’s derating data, cooling arrangement and installation requirements under the locally applicable rules before final selection.