Calculator
Estimate how much fuel a standby generator may consume and what it may cost to run — by outage duration, daily operation, or annual hours. Compare natural gas, propane, and diesel side by side.
Fuel Type
Calculation Mode
Enter how long the generator runs per day during an outage.
National average estimate: $1.20 per therm. Check your utility bill or recent delivery invoice for your local rate.
20 kW · 50% load · natural gas at $1.20 per therm
Estimated operating cost using your current generator size, load and fuel price.
Estimated hourly cost at national average prices · 20 kW at 50% load
$1.20 per therm
$2.50 per gallon
$3.80 per gallon
Comparison uses national average fuel prices.
Planning estimate only. Fuel consumption varies with load profile, ambient temperature, altitude, engine age, and maintenance condition. Monthly and annual projections in daily mode assume the generator runs every day at the selected hours — actual outage frequency in your area will determine realistic annual costs.
Where utility service is available, natural gas typically offers the lowest hourly operating cost and eliminates the need to manage an on-site fuel supply. Supply interruptions during major storm events are uncommon but possible in some areas — confirm reliability with your local utility.
Propane can be stored on-site for months without degradation, offering fuel security regardless of utility availability. It typically costs more per hour than natural gas, and tank capacity limits total runtime — plan for timely refills during extended outages.
Diesel generators can maintain strong efficiency under heavier loads and provide a reliable fuel supply independent of utilities. Diesel should be treated with a fuel stabilizer every 6–12 months when stored in standby tanks to prevent degradation.
Fuel cost during an outage depends on your generator's rated size, the electrical load it is carrying, the fuel type, your local price, and how long the outage lasts. This calculator estimates cost per hour and per day based on the load fraction you select. Most outages last a few hours; extended outages of several days are less frequent but do occur in severe storms or major grid events. Enter your local fuel price and expected outage hours for the most relevant estimate.
Natural gas is generally less expensive per equivalent energy unit than propane or diesel for residential customers in the US, and it is delivered continuously through the utility distribution system — there is no tank to fill and no delivery surcharge. Propane and diesel prices are more volatile and include delivery costs. The calculator uses approximate national average prices as defaults; your actual utility rate or supplier price may differ significantly, especially for propane in rural areas.
The estimates are based on a linear consumption model fitted to published specification sheets for residential standby generators across common size ranges. Actual consumption varies by generator model, engine calibration, ambient temperature, altitude, fuel quality, and the specific mix of resistive and inductive loads running. Use these figures for planning purposes. For precise fuel budgeting, consult your generator's owner's manual for its rated consumption at specific load conditions.
Annual fuel cost depends almost entirely on how many hours the generator actually runs — including both outage hours and weekly automatic exercise cycles. Most standby generators are programmed to exercise for 15–30 minutes once a week, adding roughly 13–26 hours per year. The calculator can estimate annual cost directly when you enter expected hours per year. Low-use standby generators in areas with infrequent outages typically have modest annual fuel costs.
Yes, but the relationship is not simply linear. Larger generators have higher fixed fuel overhead at any given load fraction because they drive larger cooling systems, governors, and ancillary equipment regardless of electrical demand. A generator that is significantly oversized for its connected load may operate at a low load fraction, which tends to be less efficient per kilowatt-hour delivered. Matching generator size reasonably to the actual load helps overall efficiency, though this should be balanced against having adequate capacity for motor starting surges.
For any estimate you intend to act on, always replace the default with your actual price. For natural gas, use the per-therm rate shown on your gas bill. For propane and diesel, use your current supplier's delivered price per gallon. National average defaults are approximate planning figures — local prices can be significantly higher or lower, particularly for propane in rural areas or diesel in regions with supply constraints.
