Estimate the natural gas fuel cost to run a generator and compare it with the same energy from the grid.
Table of contents
How to use our Natural Gas Generator Fuel Cost Calculator
- Enter Gas use (cubic feet per hour) from your generator manual, spec sheet, or load chart for the load you expect.
- Enter Run time (hours), Natural gas price ($ per selected unit), and the matching Price unit on your gas bill.
- Enter Average electric load (kW) so the calculator can show Generator fuel cost per kWh and compare it with grid electricity.
- Open Advanced options only if you want to change Gas heat content (Btu per cubic foot) or Grid electricity price ($ per kWh).
- Check the output by asking if Gas volume used equals your gas use rate times your run time; if it does not look right, recheck the generator load chart and bill unit.
Definitions
Gas use (cubic feet per hour): The volume of natural gas the generator burns each hour at a chosen load level.
Therm: A natural gas energy unit used on many gas bills. This calculator uses 100,000 Btu per therm.
Ccf: One hundred cubic feet of natural gas.
Mcf: One thousand cubic feet of natural gas.
Gas heat content (Btu per cubic foot): The heat energy in one cubic foot of natural gas. It is used to convert gas volume into therms.
Average electric load (kW): The average electric power the generator supplies during the run.
Generator fuel cost per kWh: Fuel cost divided by the electric energy made by the generator.
Generator fuel cost minus grid cost: Generator fuel cost minus the estimated cost of buying the same kWh from the grid. Positive means generator fuel costs more; negative means it costs less.
Common mistakes and quick fixes
Mistake: Using a full-load number in Gas use (cubic feet per hour) when the generator will run at a lighter load.
Fix: Use the gas-use row from the generator load chart that best matches your expected load.
Mistake: Entering a monthly bill total in Natural gas price ($ per selected unit) .
Fix: Enter the price per therm, per Ccf, or per Mcf, then match it with Price unit on your gas bill .
Mistake: Picking Therm ($/therm) in Price unit on your gas bill when the bill price is listed per Ccf or Mcf.
Fix: Choose the exact unit shown on the bill so the conversion is handled correctly.
Mistake: Putting generator size in Average electric load (kW) instead of the average load you will actually use.
Fix: Estimate the running load, not the maximum nameplate rating, for a better Generator fuel cost per kWh .
Mistake: Ignoring Gas heat content (Btu per cubic foot) when your gas bill lists a therm factor or heat-content value.
Fix: Replace the default with the bill value if you have it.
Mistake: Reading a negative Generator fuel cost minus grid cost as an error.
Fix: A negative value means the estimated generator fuel cost is lower than the entered grid-cost estimate.
Limitations & Key Assumptions / Boundary Conditions
- The result is fuel cost only. It does not include generator maintenance, oil changes, installation, standby losses, repairs, or wear.
- Generator gas use changes with load, model, altitude, temperature, and maintenance. A load-chart value from the generator maker is usually better than a rough size estimate.
- The default Gas heat content (Btu per cubic foot) may not match your local gas supply. Use your bill value if it lists one.
- Gas bills can include fixed monthly charges, taxes, demand charges, riders, or delivery fees. Include only per-unit charges in Natural gas price ($ per selected unit) if you want the per-run fuel estimate.
- Grid electricity price ($ per kWh) is only for comparison. Use an all-in rate if you want the comparison to reflect delivery and supply charges.
- The calculator does not check whether your gas meter, regulator, pipe size, or gas service can safely supply the generator.
- If Average electric load (kW) is too high or too low, Generator fuel cost per kWh and the grid comparison will be misleading even when the fuel-cost total is correct.
Methodology
How the fuel cost is calculated
The calculator starts with the generator gas-use rate and the run time. It first finds total gas volume, then converts that gas volume into therms using the entered heat content.
gas_cf = gas_cf_per_hour * run_hours
therms_used = gas_cf * heat_content_btu_per_cf / 100000
If the bill price is already per therm, that value is used directly. If the bill price is per Ccf or Mcf, the calculator converts that bill unit into a therm-based price using the same heat-content setting.
if unit = therm: price_per_therm = gas_price
if unit = ccf: price_per_therm = gas_price / (100 * heat_content_btu_per_cf / 100000)
if unit = mcf: price_per_therm = gas_price / (1000 * heat_content_btu_per_cf / 100000)
Total fuel cost is the therms used times the price per therm. Fuel cost per hour divides that total by run time.
total_fuel_cost = therms_used * price_per_therm
cost_per_hour = total_fuel_cost / run_hours
How the grid comparison is calculated
The calculator estimates generated electricity as average load times run time. It then divides fuel cost by that energy to find cost per kWh. The grid comparison is signed, so a negative difference is kept instead of changed to zero.
generated_kwh = avg_load_kw * run_hours
cost_per_kwh = total_fuel_cost / generated_kwh
grid_cost = generated_kwh * grid_price_per_kwh
generator_minus_grid = total_fuel_cost - grid_cost
Mini example
For a generator that uses 150 cubic feet per hour for 24 hours, gas volume is 3,600 cubic feet. With heat content of 1,036 Btu per cubic foot, gas used is 37.296 therms. At $1.50 per therm, total fuel cost is $55.94. If the average load is 10 kW, the generator makes 240 kWh, so fuel cost is about $0.233 per kWh. At a grid rate of $0.17 per kWh, the same energy would cost $40.80, so the generator fuel estimate is $15.14 higher than grid electricity.
Calculation choices
Blank or invalid required fields stop the calculation because zero gas use, zero run time, zero load, or a missing price would make the main outputs unreliable. The heat-content field has an editable default. A blank grid price can be skipped in the calculator behavior, but a non-blank invalid grid price should be corrected before using the grid comparison.