A natural gas generator’s therm use equals its gas consumption in cubic feet multiplied by the gas heat content in Btu per cubic foot, then divided by 100,000. Use the result to estimate fuel-related billing charges, while keeping the generator’s operating hours and electrical load explicit.
Open the Natural Gas Generator Consumption Calculator.
A therm measures fuel energy, not gas volume
A therm is an energy unit equal to 100,000 Btu. Cubic feet measure gas volume. Converting between the two requires the heat content associated with the supplied gas, so a cubic-foot reading and a therm reading are not interchangeable.
The embedded calculator produces cubic feet for the generator load, efficiency, and time entered. Convert that result to therms using your utility’s relevant conversion basis. If you already have a documented generator gas-volume rate, you can use that rate directly instead of estimating consumption from electrical efficiency.
The US Energy Information Administration’s natural-gas unit guide identifies common gas units, including therm, Ccf, and Mcf. Check the abbreviations on both your fuel specification and utility bill before transferring any number into a cost calculation. A unit multiplier can matter much more than the last decimal place in an efficiency estimate.
Convert cubic feet into therms
Use this relationship:
Therms = gas volume in ft³ × gas heat content in Btu/ft³ ÷ 100,000
As an illustrative example, assume the generator uses 1,420 ft³ during a defined outage and the applicable gas heat content is 1,030 Btu/ft³. Fuel energy is 1,420 × 1,030 = 1,462,600 Btu. Dividing by 100,000 gives 14.626 therms.
If the operating interval was ten hours, average use was 1.4626 therms per generator-running hour. If the same volume was consumed across a full 24-hour outage with intermittent operation, the average per elapsed outage hour would be different. State which time basis you are reporting.
Use the utility’s documented billing factor where available. A convenient assumption of 1,000 Btu/ft³ makes the arithmetic simple, but it does not establish your exact supplied heat content. Do not alter the factor merely to make a prediction equal one month’s bill.
Read Ccf, Mcf, and meter multipliers correctly
Ccf represents 100 cubic feet, and Mcf represents 1,000 cubic feet in standard US natural-gas notation. Convert those units to cubic feet before using the formula, unless your utility provides a direct therm-per-unit factor.
| Reported quantity | Cubic feet represented | Example conversion at 1,030 Btu/ft³ |
|---|---|---|
| 1 ft³ | 1 ft³ | 0.0103 therm |
| 1 Ccf | 100 ft³ | 1.03 therms |
| 1 Mcf | 1,000 ft³ | 10.3 therms |
| 14.2 Ccf | 1,420 ft³ | 14.626 therms |
The table’s heat content is an illustrative assumption. It shows why treating one Ccf as exactly one therm can introduce a difference. The difference may be small for a short run but becomes more noticeable across a larger fuel total.
Check whether a meter reading or bill quantity already incorporates pressure, temperature, or heat-content adjustments. Applying a billing multiplier twice can overstate use just as easily as ignoring it can understate use. Ask the utility to explain an unfamiliar factor rather than guessing from its name.
Estimate therms from electrical output
When model-specific gas consumption is unavailable, you can develop a preliminary energy-based estimate. The required inputs are delivered electrical kWh and electrical efficiency on a compatible fuel-energy basis.
Therms = electrical kWh × 3,412 ÷ electrical efficiency ÷ 100,000
Suppose a hypothetical generator supplies an average 10 kW for twelve hours at an assumed 30% electrical efficiency. Electrical output is 120 kWh. The estimated gas requirement is 120 × 3,412 ÷ 0.30 ÷ 100,000 = 13.648 therms.
At the illustrative 1,030 Btu/ft³ heat content, that corresponds to about 1,325.05 ft³. The embedded calculator can estimate the volume using the same load, time, and efficiency assumptions. Converting its volume back to therms should agree apart from rounding.
That agreement checks unit consistency, not real-world performance. The efficiency assumption may still be unsuitable for the exact generator or loading. If the manufacturer publishes a relevant fuel rate, use that specification as the stronger model-specific starting point.
Turn therm use into an incremental gas-cost estimate
Multiply estimated therms by the applicable variable cost per therm. Include the supply and delivery components that actually vary with consumption under your tariff. Keep fixed customer charges separate because they may be payable whether the generator runs or not.
For illustration only, use 13.648 therms and an invented variable rate of $1.60 per therm. The incremental consumption-related cost is 13.648 × $1.60 = $21.8368, or about $21.84. This is not a current local price quote.
If an illustrative monthly fixed charge were $12, adding it would create a hypothetical $33.84 subtotal for that simplified bill. However, attributing all $12 to the generator would overstate its incremental outage fuel cost if the customer would pay that charge anyway. Choose the cost boundary that matches your question.
Taxes, riders, tiered rates, billing-period changes, and other tariff provisions can affect an actual bill. Use the current utility bill or tariff for the installation rather than a national average when checking a real expense. Do not compare a commodity-only therm price with another option’s all-in operating cost without identifying the difference.
Separate generator use from household gas consumption
A utility meter can record the generator, furnace, water heater, and cooking equipment together. A higher bill during an outage is not automatically a measurement of generator consumption. Weather and occupancy may change other gas uses during the same period.
If you compare meter readings, keep a log of the interval and other appliances operating. Use an existing approved submeter where the installation has one, or ask the utility or installer about suitable monitoring. Do not modify the gas meter or piping to isolate demand for a home experiment.
A before-and-after monthly comparison can provide context but is not a controlled test. Different billing days, temperatures, estimated readings, or rate changes can alter the result. For a useful audit, separate quantity differences from price differences.
The generator’s own fuel chart is often the easiest starting point for an outage-specific estimate. Multiply the appropriate rate by logged operating hours, convert volume to therms, and label the result as an estimate rather than an isolated meter measurement.
Account for exercise runs and different loads
Exercise runs add to monthly consumption, but their load may differ greatly from outage operation. Use a documented rate for the actual exercise condition when available. A no-load exercise period should not automatically inherit the same efficiency or consumption per kWh as a heavily loaded outage period.
Calculate each operating category separately:
- Exercise volume or therms per run multiplied by the number of runs.
- Outage volume or therms at the expected load and duration.
- Other documented generator tests or service runs.
Add those quantities to create a generator total for the defined reporting period. Retain the categories if you want to understand why one month differs from another. A single monthly total can conceal whether the change came from exercise scheduling or a longer interruption.
The same kW rating does not guarantee identical therm consumption across models. Generac’s fuel guidance organizes consumption by model, fuel, and loading, illustrating why exact equipment information matters. Match those conditions before using a rate in a billing estimate.
Frequently asked questions
Is one therm always 100 cubic feet?
No. One therm is 100,000 Btu, while 100 cubic feet is a volume. They correspond only when the applicable heat content is exactly 1,000 Btu/ft³.
Does natural gas consumption tell me the generator’s electrical efficiency?
Only when you also know electrical energy delivered and the gas’s compatible energy content. A fuel-volume total alone does not establish efficiency.
Can average therms per hour determine gas-pipe size?
No. The installer must assess required maximum fuel flow and pressure, pipe conditions, and other connected appliances. The therm estimate here is an energy and cost calculation.
References
- Natural Gas Generator Consumption Calculator.
- EIA guide to natural-gas measurement units.
- Generac generator fuel-consumption guidance.