24-hour outage
- Planned run time
- 24 hours
- Fuel needed · typical
- 6,066 ft³Low–high: 5,472–6,660 ft³
- Billable fuel · typical
- 62.8 thermsLow–high: 56.7–69 therms
- Operating cost · typical
- $94Low–high: $85–$103
Free fuel-use tool Model 2026.08 · Reviewed 2026-08-21
Whole House Generator Running Cost Calculator
Estimate natural-gas or propane consumption, outage fuel needs, and low, typical, and high operating cost from published OEM curves and your own fuel price.
Built for representative 10–26 kW air-cooled standby scenarios—not a live fuel quote, tank recommendation, or fuel-system design.
- Natural gas or propane
- 25%–100% load bands
- 1 hour to 14 days
01 Fuel-use planner
Estimate outage fuel and operating cost.
Select a representative size and operating range, then replace the starting fuel price with your current bill or supplier quote.
Change first: fuel, outage hours, and local price.
Planned outage scenarios
Compare total generator run hours using your selected 22 kW curve, natural gas, moderate-to-heavy load, and $1.50 per therm. These estimates do not change your main runtime of 24 hours.
These are fuel budgets, not safe maximum runtimes. Allow for manual-required shutdowns, inspections and maintenance; 24, 48 or 72 planned run hours do not promise uninterrupted service.
48-hour outage
- Planned run time
- 48 hours
- Fuel needed · typical
- 12,132 ft³Low–high: 10,944–13,320 ft³
- Billable fuel · typical
- 125.7 thermsLow–high: 113.4–138 therms
- Operating cost · typical
- $189Low–high: $170–$207
72-hour outage
- Planned run time
- 72 hours
- Fuel needed · typical
- 18,198 ft³Low–high: 16,416–19,980 ft³
- Billable fuel · typical
- 188.5 thermsLow–high: 170.1–207 therms
- Operating cost · typical
- $283Low–high: $255–$310
How long can a whole house generator run continuously?
No single number of hours applies to every standby generator. Actual operation depends on the exact model, load, fuel supply, environmental conditions, and inspection and maintenance requirements.
Use this calculator to budget fuel and cost for your planned run hours—not to establish a mechanical limit, safe operating time, gas-system capacity, or guaranteed service interval. The 1–336-hour input range is a planning range, not permission to run without stopping.
- Exact model and owner’s manual. Follow the instructions for your model and serial range, including required shutdowns.
- Load level. Higher demand changes fuel consumption; the selected band is a planning assumption, not measured demand.
- Fuel supply. Natural-gas service must remain available; propane operation depends on available fuel and replenishment.
- Inspection, oil and maintenance. Plan for the checks and service your manual requires during an extended outage.
- Weather and operating conditions. Temperature, airflow and site conditions can constrain operation.
- Fuel-system capacity. Have a qualified installer or fuel supplier check the exact model’s requirements against the site’s supply; fuel quantity alone does not establish capacity.
The Generac 9–22 kW owner’s manual provides model-specific examples, not a universal limit. Reviewed September 14, 2026. Plan inspection, maintenance and longer-term service costs alongside the outage fuel budget.
02 Model-derived examples
Whole house generator fuel use for a 24-hour outage.
Every row uses the moderate-to-heavy 50% / 62.5% / 75% load range and the same pure model as the live tool. Costs use the editable starting placeholders shown below, not a market-price claim.
| Representative size | Natural gas use | Natural gas cost | Propane use | Propane cost |
|---|---|---|---|---|
| 10 kW50%–75% load | 2,556 ft³2,352–2,760 ft³ | $40$37–$43 | 26.5 gal22.6–30.5 gal | $66$56–$76 |
| 14 kW50%–75% load | 4,860 ft³4,536–5,184 ft³ | $76$70–$81 | 49.2 gal42.2–56.2 gal | $123$106–$140 |
| 18 kW50%–75% load | 4,362 ft³3,936–4,788 ft³ | $68$61–$74 | 46.9 gal39.6–54.2 gal | $117$99–$136 |
| 22 kW50%–75% load | 6,066 ft³5,472–6,660 ft³ | $94$85–$103 | 68.9 gal60.7–77.2 gal | $172$152–$193 |
| 26 kW50%–75% load | 5,382 ft³4,512–6,252 ft³ | $84$70–$97 | 58.9 gal49.4–68.3 gal | $147$124–$171 |
Starting price policy. Natural gas begins at $1.50/therm and propane at $2.50/gallon solely so the examples are reproducible. Replace either value with a current bill or supplier quote.
03 Transparent formula
From OEM fuel curve to outage operating cost.
- 1Select a representative OEM anchor row.
The chosen representative size supplies published 50% and 100% natural-gas and propane rates.
- 2Apply the declared load band.
A visible straight line evaluates the low, typical, and high percentages from 25% through 100%.
- 3Multiply rate by runtime.
Cubic feet or gallons per hour become the total outage fuel quantity for 1–336 hours.
- 4Convert units and apply your price.
EIA factors convert natural gas to therms and both fuels to MMBtu before the editable marginal price creates cost.
Exact-model boundary: nominal kW alone does not make two generators consume the same fuel. Use the installed unit’s current specification and measured run history whenever available.
Fuel rate curve
OEM anchors×load band
rate×hours×priceoperating cost
Low / typical / high stays visible at every step04 Bill and tank boundaries
Natural-gas therms and propane gallons answer different planning questions.
Natural gas: cubic feet → therms → cost.
OEM sheets publish cubic feet per hour. The calculator uses the EIA factor of 1,036 Btu per cubic foot and 100,000 Btu per therm so a $/therm bill price can be applied consistently.
Utility fixed charges, taxes, demand structures, and local heat content stay outside the displayed fuel-only range.
Propane: gallons consumed, not a tank recommendation.
The result reports gallons used over the selected hours. Tank nameplate capacity is different from usable outage fuel and does not prove adequate cold-weather vaporization.
Confirm fill limits, reserve, tank wetted surface, regulator, vaporization, pipe size, delivery access, and local requirements with the supplier and qualified installer.
05 Fuel comparison
Compare propane and natural gas on the same size, load, and runtime.
A fair operating-cost comparison holds the scenario constant and changes the exact model’s fuel curve plus local price. Fuel availability, outage resilience, tank logistics, gas-service capacity, installation scope, and maintenance remain separate decisions.
- Representative generator size
- Operating-load range
- Total runtime hours
- Excluded fixed and installation costs
- Exact installed-model fuel curve
- Current utility marginal $/therm
- Current delivered propane $/gallon
- Supplier and fuel-system capacity review
06 Evidence register
OEM curves, EIA conversions, and explicit internal assumptions.
Direct manufacturer documents own fuel-consumption anchors. EIA owns displayed energy conversions. Generator Budget owns the load bands, editable starting prices, validation limits, and rounding.
Generac
Open source 60 Hz Air-Cooled Generators, 9–22 kW — Owner’s Manual
Model-specific runtime instructions, extended-outage shutdowns for maintenance, oil checks, and environmental operating limits (printed pages 2, 7–8, 11, 19 and 23–25).
Generac
Open source 10/14/18 kW Air-Cooled Standby Generator Specification Sheet
Model-specific 50% and full-load natural-gas ft³/hour and propane gallons/hour anchors for the 10, 14, and 18 kW rows.
Generac
Open source 16/20/22 kW Guardian Standby Generator Specification Sheet
Model-specific 50% and full-load natural-gas ft³/hour and propane gallons/hour anchors for the representative 22 kW row.
Generac
Open source 26 kW Guardian Residential Standby Generator Specification Sheet
Model-specific 50% and full-load natural-gas ft³/hour and propane gallons/hour anchors for the representative 26 kW row.
Cummins
Open source RS25 Quiet Connect Series Specification Sheet
A published 25%, 50%, 75%, and full-load curve for natural gas and propane, used to validate the page's explicit load-band interpolation method and cross-manufacturer boundary.
U.S. Energy Information Administration
Open source British Thermal Units (Btu) — Sample Conversion Factors
1 cubic foot of natural gas = 1,036 Btu, 1 therm = 100,000 Btu, and 1 gallon of propane = 91,452 Btu for displayed therm and energy conversions.
Review all 5 running-cost assumption groupsCurrent values, evidence boundaries, references, and review datesOpen
| Model field | Current value or rule | Evidence boundary | Reference and review |
|---|---|---|---|
| Representative OEM fuel curves | 10 kW: NG 98/132 ft³/hr; LP 0.94/1.6 gal/hr at 50%/100% load · 14 kW: NG 189/243 ft³/hr; LP 1.76/2.92 gal/hr at 50%/100% load · 18 kW: NG 164/235 ft³/hr; LP 1.65/2.87 gal/hr at 50%/100% load · 22 kW: NG 228/327 ft³/hr; LP 2.53/3.9 gal/hr at 50%/100% load · 26 kW: NG 188/333 ft³/hr; LP 2.06/3.63 gal/hr at 50%/100% load | External referenceEach row preserves the cited model's published anchors. Nominal kW alone does not make two brands or revisions consume the same fuel; the exact installed model specification replaces this planning row. | Generac: 10/14/18 kW Air-Cooled Standby Generator Specification Sheet ↗Generac: 16/20/22 kW Guardian Standby Generator Specification Sheet ↗Generac: 26 kW Guardian Residential Standby Generator Specification Sheet ↗Reviewed 2026-08-21 · Model 2026.08 |
| Operating-load ranges | Light-to-moderate load: 25% / 37.5% / 50% · Moderate-to-heavy load: 50% / 62.5% / 75% · Heavy-to-full load: 75% / 87.5% / 100% | External reference + internal planning assumptionThe Generac rows publish 50% and 100% anchors; Generator Budget applies a visible straight-line curve from 25% through 100%. Cummins publishes quarter-load increments that validate the method shape, while each selected size retains its own OEM anchors. | Generac: 10/14/18 kW Air-Cooled Standby Generator Specification Sheet ↗Generac: 16/20/22 kW Guardian Standby Generator Specification Sheet ↗Generac: 26 kW Guardian Residential Standby Generator Specification Sheet ↗Cummins: RS25 Quiet Connect Series Specification Sheet ↗Reviewed 2026-08-21 · Model 2026.08 |
| Fuel-unit and energy conversion | Natural gas 1,036 Btu/ft³; 100,000 Btu/therm; propane 91,452 Btu/gallon. | External referenceEIA factors support comparable therm and MMBtu displays. Actual delivered heat content varies, so utility billing and supplier documents control a project-specific conversion. | U.S. Energy Information Administration: British Thermal Units (Btu) — Sample Conversion Factors ↗Reviewed 2026-08-21 · Model 2026.08 |
| Editable planning fuel price | Starting placeholders: $1.50/therm natural gas and $2.50/gallon propane. | Internal planning assumptionThese are editable starting placeholders rather than current national averages. The visitor should enter the marginal commodity and delivery price from a current utility bill or propane quote; fixed charges and taxes stay outside the model. | Internal planning assumptionReviewed 2026-08-21 · Model 2026.08 |
| Displayed result rounding | Natural-gas rates to 0.1 ft³/hour, propane rates to 0.01 gallon/hour, total gas to 1 ft³, total propane and therms to 0.1 unit, energy to 0.01 MMBtu, hourly cost to cents, and outage cost to whole dollars. | Internal planning assumptionRounding communicates planning precision and happens after raw quantity, conversion, and cost calculations. It does not turn the representative curve into an exact bill forecast. | Internal planning assumptionReviewed 2026-08-21 · Model 2026.08 |
Review owner: Generator Budget project maintainer. Reviewed 2026-08-21. Recheck at least quarterly and whenever an OEM curve, EIA conversion, supported size, or unit rule changes. Send a model correction or source update.
08 Plain-language answers
Whole house generator fuel and running-cost questions.
These answers use the same representative curves and unit boundaries as the calculator.
01How long can a whole house generator run continuously?
There is no single continuous-runtime limit for every standby generator. The exact model and owner's manual, load, natural-gas availability or propane supply, weather, and required inspections, oil checks and maintenance determine the operating boundary. Follow the manual's shutdown and service instructions, and have fuel-system capacity checked by a qualified installer or fuel supplier. This calculator estimates fuel and cost for planned run hours; it does not guarantee mechanical limits, safe runtime, gas-system capacity or service intervals.
02How much does it cost to run a whole house generator for 24 hours?
For this page's default 22 kW natural-gas scenario, moderate-to-heavy load, and editable $1.50 per therm starting price, the modeled 24-hour operating cost is $85–$103, with $94 typical. Enter the marginal fuel price from a current bill or supplier quote because fixed charges, taxes, delivery fees, and local tariffs are outside the model.
03How much natural gas does a 22 kW whole house generator use?
The representative 22 kW row uses a published 228 ft³/hour natural-gas rate at 50% load and 327 ft³/hour at full load. The default 50%–75% operating range therefore shows 228–277.5 ft³/hour and 5,472–6,660 ft³ over 24 hours. Use the exact installed-model specification for project planning.
04How much propane does a whole house generator use per hour?
Propane use depends on generator model and operating load. In the representative 22 kW row, the published anchors are 2.53 gallons/hour at 50% load and 3.90 gallons/hour at full load. The live calculator turns the selected load range and runtime into low, typical, and high gallons rather than applying one universal rate.
05What generator load percentage should I use?
Choose light-to-moderate for an essentials-focused plan, moderate-to-heavy for broader household loads, or heavy-to-full for a high-demand scenario. These are planning bands spanning 25%–100%, not an electrical load calculation. Use measured data, transfer-switch controls, or the sizing calculator to improve the load assumption.
06Can this calculator tell me what propane tank size I need?
The propane result shows gallons consumed during the selected runtime, but it does not recommend tank capacity. Usable fill, reserve policy, temperature, wetted surface, vaporization, regulator and pipe sizing, delivery access, and local requirements all affect the fuel-system decision. Review the exact generator manual with the propane supplier and qualified installer.
07Is propane or natural gas cheaper for a standby generator?
The answer depends on the exact model's fuel curve and the prices available at the property. Enter dollars per therm for natural gas or dollars per gallon for propane, then compare like-for-like size, load, and runtime scenarios. The page estimates fuel consumption and operating cost; it does not decide fuel availability, installation scope, resilience, or total ownership cost.