Reading a Generator Spec Sheet and Derating

Why this matters

The big number on the front of a standby generator's brochure is its best case: the higher fuel, at the rating altitude, at the rating temperature. The house the unit is going on may be on the other fuel, a mile up, in a summer that runs hot. A load sheet compared against the brochure number instead of the derated one produces a unit that carries the house fine in the showroom's conditions and comes up short on the afternoon it matters. This card reads the spec sheet and applies the derates. The load method itself is 'Generator Sizing for Residential'; fuel conversions are 'Natural Gas and Propane Heat Content and Generator Consumption'.

The sheet

The block below is illustrative, written for this example. Every number you size with comes from the sheet for the model and fuel you are quoting.

SPEC SHEET (illustrative unit)
RATED OUTPUT          LP 22.0 kW     NG 19.5 kW
  120/240 V, 1 phase, 60 Hz, PF 1.0
  Ratings at 77 F and 500 ft
CONSUMPTION           LP gal/h       NG ft3/h
  1/2 load              2.4            205
  Full load             3.6            310
FUEL INLET PRESSURE, in. w.c.
  NG 5 to 7            LP 10 to 12
DERATE NOTES
  Altitude: 3% per 1,000 ft above 500 ft
  Temperature: 1% per 10 F above 77 F
  Apply both; derates add

Reading the rating lines

Two kW figures, one per fuel. The propane rating is higher than the natural gas rating on most spark-ignited standby units, here 22.0 kW against 19.5 kW, a gap of 2.5 kW, about 11 percent of the propane figure. The reason is mostly the fuel-air mixture. Each cylinder fills with a fixed volume of mixture, and a stoichiometric propane-air mixture carries roughly 5 percent more energy per cubic foot than a natural gas-air mixture. Natural gas also arrives at lower supply pressure, which leaves the fuel system less room to make up the difference. The result is a lower ceiling on natural gas, and the maker publishes it as its own line.

The rating conditions. "Ratings at 77 F and 500 ft" is the basis for both kW figures. Every derate on the sheet is measured from those two numbers. A sheet that gives no rating basis is not giving you enough to size with; get the maker's full spec sheet rather than the brochure.

Power factor 1.0. Most single-phase residential standby units are rated at unity power factor, so kW equals kVA on the sheet. Three-phase commercial sets are usually rated in kW at 0.8 power factor, with a larger kVA figure beside it. Do not mix the two.

Standby rating. This is the output for the duration of an outage. Commercial sets also publish a prime rating, lower, for units that run as the main source. A residential sheet that gives one figure is giving the standby rating.

Consumption and inlet pressure belong to the fuel lines above them: the 2.4 and 3.6 gallons per hour are at half and full propane load, 11.0 and 22.0 kW. The inlet pressure range is read at the unit's test port, static and at full load, per 'How to Measure Gas Pressure at the Generator, Static and Under Load'. A supply that falls below the minimum at full load costs output whatever the derate arithmetic says.

Why altitude and heat cut output

A naturally aspirated engine makes power from the mass of air it draws into each cylinder. Thinner air means less oxygen per intake stroke, less fuel the engine can burn with it, and less power. Air density falls roughly 3 percent per 1,000 feet near sea level, which is why altitude derates are commonly in that neighborhood. Hot air is also less dense than cool air, so the same effect, smaller, applies above the rating temperature. Heat also loads the cooling system: an air-cooled unit pulling in 100 F air has less temperature difference to shed heat across, and some makers' temperature derates include that.

The alternator is affected far less than the engine. The derate is mainly an engine limit, which is why it applies to kW output and why the maker states it as one percentage that covers the unit.

Worked: the same unit at sea level and in the mountains

A house on natural gas, with the same 22.0 / 19.5 kW unit from the sheet. The site walk ('How to Size a Standby Generator From a Site Walk') produced a running load of 14.5 kW for everything the customer wants on generator, with the largest motor start checked separately per 'Motor Starting kVA and the NEC Code Letter'.

At a coastal site, 200 ft, design hot day 90 F.

• Altitude: 200 ft is below the 500 ft basis, so no altitude derate.
• Temperature: 90 minus 77 is 13 F above the basis. At 1 percent per 10 F, that is 1.3 percent.
• Natural gas output: 19.5 kW times (1 minus 0.013) is 19.5 times 0.987, which is 19.2 kW.
• Margin over the 14.5 kW running load: 19.2 minus 14.5 is 4.7 kW, about 24 percent of 19.2.

At a mountain site, 6,500 ft, design hot day 95 F.

• Altitude: 6,500 minus 500 is 6,000 ft above the basis, six thousands at 3 percent each, 18 percent.
• Temperature: 95 minus 77 is 18 F above the basis, 1.8 percent.
• The sheet says the derates add: 18 plus 1.8 is 19.8 percent.
• Natural gas output: 19.5 times (1 minus 0.198) is 19.5 times 0.802, which is 15.6 kW.
• Margin: 15.6 minus 14.5 is 1.1 kW, about 7 percent of 15.6.

Check it from the other side. 15.6 divided by 19.5 is 0.80, an output loss of about a fifth, which matches 19.8 percent. The coastal 19.2 and mountain 15.6 differ by 3.6 kW on the same unit, on the same fuel.

If the sheet had said the derates multiply. 0.82 times 0.982 is 0.805, giving 15.7 kW, 0.1 kW more. The difference is small at these numbers, but use whichever method the sheet names, not the one that gives the bigger answer.

The call at the mountain site. A 1.1 kW margin is too thin to carry a 14.5 kW running load through a compressor start on top of it, and the maker's starting capability falls with engine output too unless the sheet says otherwise. The options, in order of what they cost the customer in comfort: a larger unit whose derated natural gas output clears the load with real margin, a load management module that holds large loads off during a start, or a smaller essential-load scope. Propane would add output: 22.0 times 0.802 is 17.6 kW, a 3.1 kW margin over 14.5. Whether that justifies a tank is a fuel conversation, not a derate one.

Why the derated figure is the one the code reads. NEC 702.4, as adopted by your jurisdiction (its subsection numbering has moved between editions), in substance requires that where automatic transfer equipment is used, the standby source can carry the full load it will pick up, or that load management limits what it picks up. An inspector or engineer reviewing that calculation at elevation compares the load against the output the unit can actually make at the site, which is 15.6 kW here, not the 19.5 kW printed on the front of the sheet. Write the derated figure, the rating basis and the derate arithmetic on the load sheet so the reviewer can follow it.

How a sea-level sizing fails in the mountains

The failure is quiet. Sized against the 19.5 kW natural gas rating, the mountain unit looked like it had 5.0 kW of room. Installed, it carries the house on an ordinary evening, because ordinary evenings rarely put 14.5 kW on it at once. Then a hot afternoon brings the air conditioner, the well pump and the refrigerator on together after a transfer. The engine cannot make 14.5 kW plus a start at 6,500 ft and 95 F, frequency sags, the voltage regulator follows, and either the compressor stalls on its overload or the controller trips on underfrequency or overload. The customer sees a generator that works most of the time, which is the hardest kind of complaint to diagnose after the fact, and the shop's load sheet shows a margin that never existed.

The tell, when it lands on a service call: a unit at elevation that drops out only when the big loads coincide, with a clean fuel pressure log and a healthy engine. Pull the original sizing and look for a sea-level rating.

Other places the sheet hides a condition

• The rating is on the higher fuel. A brochure heading reads "22 kW" and the house is on natural gas. The natural gas line is the one to use.
• The rating temperature. Some sheets rate at a cooler basis than others. Two units with the same headline kW can derate differently on the same site.
• The altitude threshold. "Above 500 ft" and "above 1,000 ft" thresholds change the arithmetic at moderate elevation. Read the threshold, not just the percentage.
• Natural gas line pressure. The rating assumes the supply holds the maker's inlet pressure at full load. A supply at the bottom of the range, or below it, costs output on top of any derate.
• An older model's sheet. Ratings and derates change across model years. Download the sheet that matches the serial plate on the unit.

Verifying the derated number on site

A derate is a prediction. The check is a load test at the site: resistive load stepped to the derated figure, with frequency and voltage held within the maker's stated regulation and gas pressure inside the range at that load. At the mountain site, that means the unit should hold 15.6 kW on natural gas on a warm day; asking it for 19.5 kW proves nothing except that the derate was real. What that kind of test does and does not prove is in 'What a Load Bank Test Proves and What It Does Not', and a motor start is tested separately at commissioning.

References

• Generator maker's full specification sheet for the model and serial in question: ratings, rating basis, consumption, inlet pressure and derate notes.
• NFPA 70, National Electrical Code, 702.4 (optional standby capacity), as adopted by your jurisdiction; check the current edition.
• See related: Generator Sizing for Residential; How to Size a Standby Generator From a Site Walk; Natural Gas and Propane Heat Content and Generator Consumption; What a Load Bank Test Proves and What It Does Not.