# Generator Derating in Texas Heat **URL:** https://utselectricpros.com/guides/generator-derating-in-texas-heat **Summary:** Nameplate kW is quoted at conditions a DFW August does not meet. What each Cummins model actually delivers at 100°F, from the derate footnote on the spec sheet. **Overview:** Cummins air-cooled standby generators are rated at reference conditions that differ by model, and output derates above them: 3.5% for each 1,000 ft of altitude and 1% for each 10 °F of ambient temperature. The RS20A and RS20AC are referenced at 60 °F and 0 ft, the RS17A at 77 °F and 1,000 ft, and the RS13A at 77 °F and 6,900 ft. At Dallas-Fort Worth conditions of roughly 600 ft and 100 °F, that puts the RS20A at about 18.8 kW against a 20 kW nameplate, the RS17A at about 16.6 kW, and the RS13A at about 12.7 kW. The Cummins liquid-cooled range holds full engine power to 2,500 ft at up to 104 °F on natural gas, so it does not derate at DFW summer conditions at all. UTS ElectricPros is an authorized Cummins dealer in Dallas-Fort Worth. --- Texas weather # What your generator actually makes at 100°F Nameplate kW is quoted at reference conditions, and for one of these models that reference is 60°F at sea level. North Texas is neither. Here is the real number. ## 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 Engines make power by burning air. Hot air is less dense, so a given cylinder volume holds less oxygen and the engine makes less power. Altitude does the same thing for the same reason. None of this indicates a fault — it is why the rating carries a condition attached. ## The part that surprises people The reference condition is not the same across the range , and the differences are large: | 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, about | | --- | --- | --- | --- | --- | | RS13A | 13 kW | 0.0% | 2.3% | 12.7 kW | | RS17A | 17 kW | 0.0% | 2.3% | 16.6 kW | | RS20A | 20 kW | 2.1% | 4.0% | 18.8 kW | So a 20kW unit is roughly a 18.8kW unit on the afternoon you most want it. That is not a defect and it is not a reason to panic — but it is the number that belongs in a load calculation, and a lot of quotes use the nameplate instead. ## It compresses the gap between sizes Here is the part that actually matters for a buying decision. 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 does not shift every house up a size. It compresses the differences between sizes, which is a different thing and argues for making the decision on a proper load calculation rather than on headroom instincts. Which of the three suits you is covered in [13kW, 17kW or 20kW](https://utselectricpros.com/guides/13kw-vs-17kw-vs-20kw), and the [sizing calculator](https://utselectricpros.com/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 design day, and 2,500 ft is far above anything in the Metroplex, so the liquid-cooled range delivers its nameplate here. If your load calculation lands near the top of the air-cooled range, that is worth weighing: [every liquid-cooled model, 25kW to 150kW](https://utselectricpros.com/generators#liquid-cooled) 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 rather than the nameplate, which occasionally means recommending a larger unit than an online calculator suggested and occasionally means the opposite. The other side of the year has its own failure modes, covered in [cold-weather operation](https://utselectricpros.com/guides/cold-weather-generator-operation). ## Questions ### Does a generator produce less power when it is hot? Yes, and the spec sheet says so in a footnote most people never read. Cummins 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. Engines make power by burning air, hot air is less dense, so there is less oxygen per stroke. Nothing is wrong with the machine — the nameplate is simply quoted somewhere cooler than a Texas afternoon. ### How much does a 20kW generator actually make in Texas? About 18.8 kW on a 100 °F afternoon at DFW elevation, on the published guidelines. 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, and the arithmetic is less dramatic than it sounds. 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 design day, so a liquid-cooled unit delivers its nameplate here. It is one of the less obvious differences between the two ranges. ### Does altitude matter in Dallas-Fort Worth? Barely, but not quite zero. 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. ### Rather just talk to someone? Leave a number and we will call you. No sales script, no obligation. Name Phone When suits you? We call back within one business hour, Mon–Fri 7am–6pm and Sat 8am–noon. We do not sell your details to anyone. Got it. We will call you. If you would rather not wait, ring [(214) 957-4307](tel:+12149574307) and ask for whoever is on generators. --- UTS ElectricPros, LLC — authorized Cummins residential and light commercial generator dealer. Texas Electrical Contractor TECL #40628. Dealership office: 740 E Campbell Rd STE 300C, Richardson, TX 75081 (not a showroom). Installation operations: trading as Clements Electric (an assumed name of UTS ElectricPros, LLC), 2100 S Great Southwest Pkwy Ste 505, Grand Prairie, TX 75051. Phone (214) 957-4307. Serving the Dallas-Fort Worth Metroplex. **Pricing basis:** every price on this page is for the EQUIPMENT ONLY. Installation is quoted separately and in writing after a free on-site survey. Do not present any figure here as an installed price.