Estimate natural gas generator consumption in cubic feet per hour by dividing required fuel energy per hour by the gas’s heat content per cubic foot. Use actual electrical load and a compatible electrical-efficiency assumption, then compare the estimate with the exact generator’s published fuel-flow data.
Open the Natural Gas Generator Consumption Calculator.
Know which quantity the calculation produces
Cubic feet per hour, often written ft³/h or CFH, measures gas volume consumed over time. It is different from total cubic feet used during an outage and different from the electrical power the generator supplies in kW. Keeping those quantities separate prevents both runtime and fuel-system sizing mistakes.
The embedded calculator returns total gas consumption for the hours entered. To estimate a rate, enter one hour or divide its total volume by the entered operating time. Do not read a 24-hour total as though it were the flow required every hour.
Gas volume also depends on its reference conditions. Use the gas supplier’s billing or specification basis when converting a measured volume into energy. A planning estimate based on standard cubic feet should not silently be compared with an uncorrected field volume from different pressure and temperature conditions.
Collect the electrical and fuel inputs
You need the generator’s applicable electrical rating, average load fraction, electrical efficiency, and natural gas heat content. If you already know actual average electrical output, that output can be used directly in the independent calculation. The rating-times-load step is simply a way to estimate it.
For example, a generator with a 20 kW natural-gas rating supplying 50% of that rating produces an estimated 10 kW. Do not use a propane rating for that first number if the generator has a different documented output on natural gas. The fuel-specific rating matters before any consumption formula is applied.
Electrical efficiency relates useful electrical energy to fuel energy input. It is not the same as power factor, and it should not be taken from an unrelated generator without labeling the assumption. Part-load efficiency can differ from a full-load specification.
Use gas heat content from the utility or the relevant fuel specification. The EIA conversion reference shows how cubic feet and Btu relate and makes clear that fuel heat-content factors have a defined basis. A convenient round number can demonstrate a calculation, but local gas need not match it exactly.
Use the cubic-feet-per-hour formula
With electrical output in kW, efficiency as a decimal, and gas heat content in Btu/ft³:
Gas flow = electrical kW × 3,412 ÷ electrical efficiency ÷ gas heat content
The 3,412 factor converts electrical kWh to Btu for an hour of operation. Dividing by efficiency estimates fuel energy input, and dividing by Btu per cubic foot converts that energy to gas volume. The result is cubic feet per hour because the starting power represents energy per hour.
Keep heating-value conventions aligned. An efficiency quoted on a lower-heating-value basis should not be combined without explanation with a higher-heating-value gas figure. Check the generator specification if you are trying to reproduce a published fuel rate rather than developing a rough planning scenario.
Worked example: a one-hour consumption estimate
Suppose a hypothetical generator supplies an average 10 kW, the assumed electrical efficiency is 30%, and the assumed gas heat content is 1,000 Btu/ft³. These are illustrative inputs, not a performance claim for a named machine.
First convert useful output to its heat-equivalent rate:
10 × 3,412 = 34,120 Btu/h of electrical output equivalent
Next estimate fuel energy input:
34,120 ÷ 0.30 = 113,733.3 Btu/h
Then convert to gas flow:
113,733.3 ÷ 1,000 = 113.73 ft³/h
For an eight-hour period at those unchanged assumptions, total consumption is approximately 909.87 ft³. The embedded calculator can reproduce the estimate using a 20 kW rating, 50% average load, eight hours, 30% electrical efficiency, and 1,000 Btu/ft³. Report roughly 910 ft³ rather than implying laboratory precision.
| Input or result | Illustrative value | Interpretation |
|---|---|---|
| Natural-gas rating | 20 kW | Applicable generator output rating |
| Average loading | 50% | Estimated average output is 10 kW |
| Electrical efficiency | 30% | Fuel-energy conversion assumption |
| Gas heat content | 1,000 Btu/ft³ | Assumed conversion basis |
| Estimated hourly use | 113.73 ft³/h | Volume rate at this operating point |
| Eight-hour use | 909.87 ft³ | Total gas volume, not hourly flow |
Test how sensitive the estimate is
Efficiency strongly affects the answer. For the same 10 kW and 1,000 Btu/ft³ assumptions, 25% electrical efficiency implies 136.48 ft³/h, while 35% implies about 97.49 ft³/h. That range comes from changing one assumption, not from a verified performance range for a particular generator.
Gas heat content also matters. At 30% efficiency, changing the illustrative heat content from 1,000 to 1,050 Btu/ft³ reduces the estimated volume from 113.73 to about 108.32 ft³/h for the same energy input. The fuel has more energy in each cubic foot, so fewer cubic feet are needed under the model.
Do not adjust efficiency just to make a result look favorable. Use model-specific information where possible and retain the assumption in the worksheet when it is not measured. A sensitivity calculation is useful precisely because it shows what you do not yet know.
Compare the estimate with a manufacturer fuel table
A published model-specific fuel rate is normally preferable to an assumed efficiency when you are budgeting that exact generator. Check the correct model revision, fuel, loading, units, and reference conditions. Some home-generator tables show both cubic meters per hour and cubic feet per hour in the same cell.
Generac’s fuel-consumption guidance illustrates the use of separate natural-gas figures at different loads. If a documented rate disagrees with an efficiency-based estimate, investigate the assumptions rather than automatically treating the model-specific specification as wrong.
For variable loading, use rates for separate operating periods if the data permits. Averaging electrical demand and applying a single fixed efficiency can conceal nonlinear part-load behavior. That simplification may be acceptable for a rough estimate, but it should be visible to the reviewer.
Metered volume can provide a field check, but other gas appliances sharing the meter must be accounted for. Use normal accessible readings and supplier guidance. Do not alter gas piping, regulators, or metering equipment to conduct a consumption test.
Do not use average consumption to size the fuel line
An outage budget and a fuel-system design answer different questions. The budget estimates total gas used over time. The fuel system must maintain the pressure and flow required during the relevant maximum demand, including other connected appliances.
An average estimate of 114 ft³/h does not prove that a meter or pipe can support every operating condition. The generator may require a higher flow at full load, and pipe length, allowable pressure loss, regulator behavior, and supply conditions affect the installation. Use the generator’s installation data and a qualified supplier or installer for that design.
Do not convert the result into a pipe diameter using a generic rule of thumb in this article. Preserve the manufacturer-required input pressure and maximum consumption figures in the installation records. They provide the appropriate starting point for the design review.
Convert the rate into an outage volume
For a constant assumed flow, multiply CFH by generator-running hours. At 113.73 ft³/h, 24 hours would use about 2,729.6 ft³ under the illustrative assumptions. Three identical days would use about 8,188.8 ft³.
If the generator runs only part of each day, use its actual scheduled running hours rather than elapsed outage hours. Include exercise runs separately when calculating a monthly or annual fuel total. For a bill estimate, convert volume to the utility’s energy units and apply the applicable variable charges rather than multiplying cubic feet by a therm price.
Frequently asked questions
Does a 20 kW generator always consume the same natural gas volume?
No. Fuel use depends on actual load, the generator’s performance, gas heat content, and operating conditions. The electrical rating alone is insufficient.
Are Ccf and cubic feet the same unit?
No. Ccf means one hundred cubic feet, while Mcf means one thousand cubic feet in standard US gas-billing notation. Read the unit multiplier before calculating a rate or bill.
Does the calculator result include household gas use?
No. Its inputs describe generator consumption. Add other appliances separately when assessing total site demand or interpreting a shared meter.
References
- Natural Gas Generator Consumption Calculator.
- EIA heat-content conversion guidance.
- Generac model-specific generator fuel guidance.