Compare generator fuel cost with utility power cost for a load, run time, fuel price, and electric bill rate.
Advanced options
Table of contents
How to use our Generator vs Utility Power Cost Calculator
- Enter Generator load in kW for the devices you plan to power, using the running load instead of the generator's maximum rating.
- Enter Fuel used each hour and Fuel price with matching units, such as gallons and $ per gallon or therms and $ per therm.
- Enter Utility energy rate in cents per kWh and Run time to compare in hours.
- Open Advanced options if you want Effective utility cost per kWh to include Fixed utility charge spread across Usual monthly utility use.
- Check the results for sense: Generator cost per kWh should fall when load goes up with the same fuel use, and a negative Generator extra cost per hour means the generator is cheaper for the entered numbers.
Definitions
Generator load: The power your connected devices use while running, measured in kilowatts (kW).
kWh: Kilowatt-hour, the energy from using 1 kW of power for 1 hour [1].
Fuel used each hour: The generator's fuel burn rate for the load you entered, in gallons per hour or therms per hour.
Utility energy rate: The electric bill energy price in cents per kWh, not including a fixed customer charge.
Fixed utility charge: A monthly bill charge that is due even before energy use charges are added.
Effective utility cost per kWh: The utility energy rate plus the fixed monthly charge spread across your usual monthly kWh.
Generator extra cost: Generator cost minus utility cost. Positive means the generator costs more. Negative means the generator costs less.
Common mistakes and quick fixes
Mistake: Using the generator's rated size for Generator load instead of the devices actually running.
Fix: Enter the real running Generator load in kW, or add up the running watts and divide by 1000.
Mistake: Mixing units between Fuel used each hour and Fuel price.
Fix: If Fuel used each hour is gallons per hour, enter Fuel price as $ per gallon; if it is therms per hour, enter $ per therm.
Mistake: Putting Fixed utility charge into Utility energy rate.
Fix: Enter only the energy charge in Utility energy rate, then put the bill's customer charge in Fixed utility charge.
Mistake: Leaving Usual monthly utility use at a value that does not match your bill while using a Fixed utility charge.
Fix: Enter your normal monthly kWh so the Effective utility cost per kWh spreads the fixed charge over the right amount of use.
Mistake: Treating Generator extra cost for this run as a warning when it is negative.
Fix: Negative Generator extra cost for this run means the generator is cheaper than utility power for the numbers entered.
Limitations & Key Assumptions / Boundary Conditions
- The generator side is fuel-only. It does not include generator purchase price, oil, maintenance, repairs, storage, permits, noise, or emissions costs.
- The calculator assumes Fuel used each hour stays constant at the entered Generator load. Real fuel burn can change with load, generator age, altitude, temperature, and fuel quality.
- The utility side uses a simple rate model: Utility energy rate plus Fixed utility charge divided by Usual monthly utility use. It does not model tiered rates, time-of-use rates, demand charges, taxes, fees, or net metering.
- Fixed utility charges are spread across normal monthly use for a fair per-kWh comparison. If you are comparing a short outage, you may prefer Fixed utility charge set to $0 because that bill charge is usually paid either way.
- Run time to compare can be 0, which gives $0 run totals while still showing per-hour and per-kWh costs.
- Generator load and Usual monthly utility use must be greater than 0 because the calculator divides by those values.
Methodology
How the costs are calculated
The calculator first finds the fuel cost for one hour, then divides that hourly cost by Generator load to estimate Generator cost per kWh. One kW used for one hour equals one kWh [1].
fuel_cost_per_hour = fuel_burn_per_hour * fuel_price
generator_cost_per_kwh = fuel_cost_per_hour / load_kw
The utility comparison converts Utility energy rate from cents to dollars, then adds the fixed monthly charge spread across Usual monthly utility use.
effective_utility_cost_per_kwh = (utility_rate_cents * 0.01) + (fixed_monthly_charge / monthly_kwh)
utility_cost_per_hour = load_kw * effective_utility_cost_per_kwh
Run totals multiply each hourly cost by Run time to compare. The difference stays signed so cheaper generator scenarios show as negative extra cost.
cost_difference_for_run = (fuel_cost_per_hour * run_hours) - (utility_cost_per_hour * run_hours)
fuel_needed_for_run = fuel_burn_per_hour * run_hours
Worked mini-example
Suppose Generator load is 5 kW, Fuel used each hour is 0.75, Fuel price is $3.50, Utility energy rate is 17 cents per kWh, Run time to compare is 8 hours, Fixed utility charge is $30, and Usual monthly utility use is 1000 kWh. Generator fuel cost per hour is 0.75 * 3.50 = $2.625. Generator cost per kWh is 2.625 / 5 = $0.525. Effective utility cost per kWh is 0.17 + (30 / 1000) = $0.20. Utility cost per hour is 5 * 0.20 = $1.00. Generator extra cost per hour is 2.625 - 1.00 = $1.625, and Generator extra cost for this run is $13.00.