Centri field guide · Generators
How long will your propane generator run?
Calculate propane generator runtime from your tank level, tank size, and generator specifications. Plan your fuel supply for a hurricane, winter storm, or power outage.
Plan before the power goes out
Your propane generator runtime
Edit the settings below with your tank level, tank size and quantity, and generator brand and size. Then select Calculate my runtime to update the chart.
Projected tank level
Generator-only fuel use, from your current level to theoretical empty.
Loading the estimate for your selected setup.
| Load | Propane use | To 20% reserve | To 10% low fuel | To empty* |
|---|---|---|---|---|
| Light · 25% | — gal/h | — | — | — |
| Typical · 50% | — gal/h | — | — | — |
| Heavy · 75% | — gal/h | — | — | — |
| Maximum · 100% | — gal/h | — | — | — |
*Empty is a mathematical endpoint, not a recommended operating target. Plan around a reserve. The 25% curve is extrapolated from the 50% and 100% reference rates; the 75% curve is interpolated. Estimates come from calculations matching the MyPropane app. Heating, hot water, cooking, and other propane use are not included.
What does your propane generator runtime actually mean?
Generator runtime is the estimated number of hours your available fuel can support a particular electrical load. To calculate it, divide the propane available above your chosen reserve by your generator's propane consumption in gallons per hour. A bigger tank buys time; a heavier electrical load usually uses that time faster.
An outage does not come with a dependable end time. Knowing your estimated runtime helps you choose which appliances to power, decide when to request propane, and prepare a backup plan before a hurricane or winter storm interrupts deliveries.
The chart compares light (25%), typical (50%), heavy (75%), and maximum (100%) electrical load. “Typical” is a comparison label, not a measurement of your home. Each line shows what would happen if that load stayed constant. Real equipment cycles on and off, so actual tank levels will not follow a perfectly straight line.
How to calculate propane generator runtime.
You need the tank's nominal gallon capacity, its current percentage reading, and a propane fuel-use figure for your exact generator model at the intended electrical load. Use the propane rating: a dual-fuel model's gasoline or natural-gas specifications are not interchangeable with its propane specifications.
For multiple connected tanks of the same size and at the same percentage level, multiply the size of one tank by the number of tanks. Four 120-gallon tanks provide 480 gallons of nominal capacity and hold approximately 288 gallons at 60%. This calculator uses that combined capacity for the fuel and reserve estimates.
Current fuel (gal) = tank capacity × tank level ÷ 100
Usable fuel (gal) = tank capacity × max(tank level − reserve, 0) ÷ 100
Runtime (hours) = usable fuel ÷ propane consumption (gal/hour)
For example, a 500-gallon tank at 50% holds approximately 250 gallons. Keeping a 20% reserve leaves approximately 150 gallons for planning. If a manufacturer's propane specification were 2 gallons per hour at your chosen load, that would mean about 75 hours to reserve. This is a worked example, not a fuel-use claim for any generator brand.
Do not multiply by 80% a second time. A 500-gallon tank at an 80% reading already represents approximately 400 gallons. The level input accounts for the current fill; nominal capacity remains 500 gallons.
Read the reserve markers before the empty endpoint.
The chart marks 20% as a planning reserve and 10% as a low-fuel reference. These are planning markers, not universal shutdown settings or a guarantee of safe fuel delivery. A theoretical time to empty is not a recommendation to run the tank empty. Contact your supplier early enough to account for weather, access, and delivery lead time.
If heating, hot water, or cooking shares the tank, add that consumption to the generator's rate for a more complete estimate. This calculator shows generator-only use; it does not assume a household baseline or infer one from your email.
Why generator brand, size, and electrical load change propane consumption.
A generator's kW rating describes electrical output capacity, not a fixed fuel burn. Two 20 kW models may have different engines, controls, and published consumption. This tool uses the stored reference for your selected brand and size; check the estimate against your exact model's specification sheet.
Running central air conditioning, electric resistance heat, pumps, and other large loads can increase fuel use. Starting motors also create short demand peaks. Plan around the appliances you actually need and confirm their starting requirements against your generator and load-management setup.
These calculations use stored specification references for the selected brand and size as a normal-atmospheric-condition estimate. The current library does not identify a model revision or provide its original specification sheet and test conditions. Check your exact model's published specifications before relying on the result. Temperature, altitude, maintenance, fuel properties, and changing load can move actual consumption away from the reference values.
The chart uses the same calculation as the MyPropane app. The 50% and 100% scenarios use stored reference rates. The 75% scenario is interpolated between them, and the 25% scenario is extrapolated below them. Both derived estimates are labeled; they are not separately measured or published load points. Generac explains how rating, load, and fuel type affect consumption.
The physics: from liquid propane to electrical power.
In a typical stationary propane-vapor installation, liquid propane stored under pressure evaporates into vapor. A regulator supplies the engine at the required pressure. Combustion releases chemical energy, the engine converts part of it into mechanical rotation, and the alternator turns that rotation into electrical power. Heat, exhaust, friction, and electrical losses account for the rest.
Electrical power ≈ efficiency × propane flow × energy per gallon
P = η × q × H
Here, P is electrical output in kW, q is propane flow in gallons per hour, and H is the fuel's energy per gallon expressed in kWh per gallon on a consistent heating-value basis. Efficiency η includes the engine and generator losses. It changes with operating conditions, so this relationship explains the system; the calculator uses the selected brand and size's stored fuel-use reference instead of guessing a universal efficiency.
A little more physics: why the chart slopes downward.
For approximately constant consumption q and nominal tank capacity C, remaining liquid volume is V(t) ≈ V₀ − qt. The percentage reading is therefore L(t) ≈ L₀ − 100qt/C. A higher fuel rate makes the line steeper; a larger tank makes its percentage level fall more slowly at the same rate.
With changing electrical load, the fuel used is the accumulated flow over time: V(t) = V₀ − ∫q(t)dt. The four straight lines are steady-load scenarios, not a prediction of every appliance cycle. Temperature changes and gauge accuracy also affect the relationship between liquid volume and the displayed level.
Evaporation draws heat from the tank and its surroundings. In cold weather, a low liquid level or an undersized fuel system can limit the available vapor even while liquid remains. Fuel quantity alone cannot confirm that the tank, regulator, and piping can sustain a generator's demand. Have the installer or propane supplier verify the supply system for the model and local winter conditions. Learn how propane tanks and their gauges work.
How to prepare your propane generator for a hurricane or winter storm.
Treat preparation as a season-long process. The following timeline is a planning framework; use your generator's manual and qualified service provider for model-specific maintenance and operating procedures.
Service the system and establish your fuel plan.
- Arrange scheduled service before the seasonal rush. Have the battery, starting system, oil and filters, and transfer equipment checked according to the manufacturer's maintenance requirements.
- Review which circuits need backup power and ask the installer to verify propane supply capacity under the expected load and temperatures.
- Record your model, tank capacity, supplier contact, and a fuel-ordering plan. Calculate more than one load scenario and decide which nonessential loads you could reduce.
- Test carbon monoxide and smoke alarms. Plan for essential medicines, refrigeration, communications, and a place to go if backup power is unavailable.
Keep readiness current.
- Follow the prescribed exercise and maintenance schedule, review alarms or failed starts, and resolve faults before a forecast becomes urgent.
- Check tank level regularly and update the runtime estimate. Other appliances may have consumed propane since the last calculation.
- Keep required ventilation clear of vegetation and debris. Maintain safe access to the tank for deliveries.
- Keep the correct maintenance supplies available and know the service intervals for extended operation. Follow the manual rather than a generic number of hours.
Confirm fuel, load priorities, and a backup plan.
- Check the latest tank reading and contact your supplier early if more fuel is needed. A last-minute delivery may not be possible.
- Recalculate at both moderate and heavy load. Plan around time to reserve and the possibility of a longer outage, not just the storm's expected duration.
- Review the generator's operating status and manufacturer's pre-storm checks. Address faults through qualified service personnel.
- Charge essential devices, keep emergency contacts accessible, and follow local weather alerts and evacuation instructions. A generator does not make a flood-prone or evacuated property safe to occupy.
For manufacturer guidance, see Cummins' hurricane preparation advice and your specific generator's owner's manual.
What changes when you prepare a propane generator for a winter storm?
Winter can increase household propane demand at the same time that cold conditions make fuel delivery to the engine more challenging. Home heating sharing the tank shortens generator runtime, and a low tank can reduce available vaporization capacity. Start with current readings and allow room for uncertainty.
Have your service provider confirm the manufacturer's cold-weather requirements, including the appropriate oil and any model-specific starting or battery equipment. Keep intake and exhaust openings clear according to the manual, and protect access to the tank and generator from snow accumulation. Do not enclose a generator or improvise a heat source around a propane tank.
Plan for frozen roads, delayed deliveries, and outages lasting longer than expected. Follow National Weather Service winter preparation guidance alongside your fuel and generator plan.
During an outage: monitor fuel and operate safely.
Track tank level and generator operating hours. Reassess the load if consumption is faster than expected, and follow the manual's inspection, cooldown, and maintenance instructions. Stop and seek qualified assistance for faults or suspected fuel-system damage. Do not adjust regulators or gas piping to chase a longer runtime.
Portable generators must run outdoors, at least 20 feet from homes, with exhaust directed away from windows, doors, and vents. Never run one in a garage or other enclosed space, even with doors open. Use working carbon monoxide alarms and never backfeed a building by plugging a generator into a wall outlet. Fixed standby installations have manufacturer-specific clearances and local requirements; have those verified by a qualified installer. Read CPSC's generator safety guidance.
A propane tank monitor can help you check available fuel and consumption over time. Remote readings depend on the monitor's power and network path: home Wi-Fi may lose power during an outage, and cellular service can also be disrupted. Include a safe alternative way to check your fuel plan.
Common questions about propane generator runtime.
How long will a 500-gallon propane tank run a generator?
There is no single runtime without the starting level and fuel-use rate. At 80%, a nominal 500-gallon tank holds about 400 gallons. A 20% reserve leaves about 300 gallons for planning. Divide those 300 gallons by the exact model's consumption at your intended load. Shared propane appliances reduce the available operating time.
Does a 20 kW generator always use 20 kW of power?
No. Its rated output is a capacity limit under specified conditions. Connected appliances determine the actual electrical demand, subject to starting loads and system controls. Fuel use follows the engine's operating characteristics at that demand, not the nameplate number alone.
Is the lowest-load curve the best outage plan?
It is a useful scenario only if that output can support your essential loads. Check actual appliance demand, motor starting requirements, and any minimum-load or operating guidance in the manual. Do not assume a light-load runtime will cover heavy heating or cooling demand.
Can I use the estimate to decide when to buy propane?
Use time to reserve as one input alongside the forecast, supplier lead times, tank access, and other household fuel use. Compare reported residential propane prices when planning quotes, and arrange deliveries before weather makes access difficult.
Are all four load scenarios published specification values?
No. Half-load and full-load use stored reference rates. The 75% estimate is interpolated and the 25% estimate is extrapolated by calculations matching the MyPropane app. If a brand/size reference is missing or invalid, the calculator reports that it is unavailable rather than substituting another generator.