Texas weather
What your generator actually makes at 100°F
At 100°F and DFW’s roughly 600 ft elevation, the Cummins derate guideline puts a 20kW RS20A at about 18.8kW on LP and about 16.9kW on natural gas. The RS17A and RS13A lose less, and the liquid-cooled range does not derate at these conditions.
The rule, quoted
“Full rated power is available up to the reference condition for each model. Above it, derate 3.5% for each increase of 300 m (1000 ft.) and 1% for each increase of 5.5 °C (10 °F).”
Cummins Power Generation spec sheet NAS-6254-EN (A056T581), footnote 3, read 2026-09-20
Hot air is less dense, so a given cylinder volume holds less oxygen and the engine makes less power. Altitude has the same effect for the same reason.
Each model has a different reference condition
The reference condition is not the same across the range:
| Model | Full power up to |
|---|---|
| RS13A — 13kW | 25 °C (77 °F) and 2100 m (6900 ft.) |
| RS17A — 17kW | 25 °C (77 °F) and 300 m (1000 ft.) |
| RS20A — 20kW | 15 °C (60 °F) and 0 m (0 ft.) |
The RS20A and RS20AC are referenced at 60°F and sea level. The RS13A is referenced at 77°F and 6,900 ft. Same derate percentages, very different starting points — so the biggest air-cooled unit is the one that loses the most in this climate.
Run at Dallas–Fort Worth conditions
Calculated at 600 ft and 100°F, applying the published guideline to each model's own reference condition. DFW elevation runs about 430 ft in Dallas and 650 ft in Fort Worth.
| Model | Nameplate | Altitude derate | Temperature derate | Delivered on LP, about | Delivered on natural gas, about |
|---|---|---|---|---|---|
| RS13A | 13 kW | 0.0% | 2.3% | 12.7 kW | 12.7 kW (rated 13 kW) |
| RS17A | 17 kW | 0.0% | 2.3% | 16.6 kW | 16.6 kW (rated 17 kW) |
| RS20A | 20 kW | 2.1% | 4.0% | 18.8 kW | 16.9 kW (rated 18 kW) |
So on a 100°F afternoon a 20kW RS20A is roughly a 18.8kW unit on LP and a 16.9kW unit on natural gas. That derated figure, on your fuel, is the one that belongs in a load calculation.
It compresses the gap between sizes
On the nameplate, the RS20A is 3kW bigger than the RS17A. In real summer conditions that gap narrows to about 2.2kW, because the RS20A derates from a stricter reference and the RS17A barely derates at all.
Derating compresses the differences between sizes rather than moving every house up one, so the decision belongs to a load calculation. Which of the three suits you is covered in 13kW, 17kW or 20kW, and the sizing calculator gives you a range against your own house.
Liquid-cooled barely notices
Natural gas: Full engine power to 2,500 ft at up to 104°F. Above that, derate 4% per 1,000 ft and 2% per 10°C.
LP: Full engine power to 3,300 ft at up to 104°F, then the same derate.
A 104°F ceiling sits above a normal North Texas summer afternoon, and 2,500 ft is far above anything in the Metroplex, so the liquid-cooled range delivers its rating here except on the hottest days. If your load calculation lands near the top of the air-cooled range, weigh that: every liquid-cooled model, 25kW to 150kW is priced on the site. Natural gas and LP output are identical on the liquid-cooled range — there is no natural gas derate.
How we use it
We size against the derated figure for your fuel rather than the nameplate. The other side of the year has its own failure modes, covered in cold-weather operation.
Questions
Does a generator produce less power when it is hot?
Yes. Footnote 3 of the Cummins spec sheet quotes full rated power up to a stated reference condition and then derates 3.5% for each 1,000 ft of altitude and 1% for each 10 °F above it. Hot air is less dense, so the engine takes in less oxygen per stroke and makes less power.
How much does a 20kW generator actually make in Texas?
On the published guideline, a Cummins RS20A delivers about 18.8 kW on LP and about 16.9 kW on natural gas on a 100 °F afternoon at DFW elevation; its natural gas rating is 18 kW before any derate. The reason the RS20A loses more than the smaller units is that it is referenced at 60 °F and sea level, while the RS17A is referenced at 77 °F and 1,000 ft. Same derate rule, stricter starting point.
Should I buy a bigger generator because of derating?
Not automatically. A 3kW gap on the nameplate between the RS17A and the RS20A narrows to about 2.2kW in real summer conditions — so derating compresses the difference between sizes rather than shifting everything up one. What it should change is whether you size to the exact calculated load or leave headroom. We use the derated figure in the load calculation rather than the nameplate.
Do liquid-cooled generators derate in Texas too?
Effectively no, at DFW conditions. Full engine power to 2,500 ft at up to 104°F. Above that, derate 4% per 1,000 ft and 2% per 10°C. On LP it is full engine power to 3,300 ft at up to 104°F, then the same derate. That 104 °F ceiling sits above a normal North Texas summer afternoon, so a liquid-cooled unit delivers its rating here except on the hottest days.
Does altitude matter in Dallas-Fort Worth?
A little. DFW sits around 600 ft — roughly 430 ft in Dallas and 650 ft in Fort Worth. That is below the reference altitude for the RS13A and RS17A, so it costs them nothing. The RS20A and RS20AC are referenced at sea level, so the elevation does cost them about 2.1% before temperature is considered at all.
The sheet states these are guidelines: maximum wattage or current is subject to and limited by fuel Btu content, ambient temperature, altitude, engine power and condition. Figures on this page apply the published guideline arithmetically to DFW design conditions; the delivered output of any specific machine also depends on fuel BTU content and engine condition. Source: Cummins Power Generation spec sheet NAS-6254-EN (A056T581), RS13A / RS17A / RS20A / RS20AC, retrieved 2026-08-19.
We size on the derated number
The survey produces a load calculation against your actual panel, using what the machine makes here rather than what the sticker says.
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