When the power goes out, your generator becomes the lifeline for keeping your home’s essentials running—refrigerators, heaters, medical devices, and communication equipment. But one of the most important factors when shopping for a home generator is how long it can run on a single tank of fuel.
Whether your generator runs on gasoline, diesel, or propane, knowing its expected runtime can help you plan for emergencies, store the right amount of fuel, and avoid mid-storm shutdowns. So, just how long can a generator run on a full tank of liquid fuel or propane?
This guide breaks down everything homeowners need to know about generator fuel runtimes, including key differences by fuel type, generator size, load percentage, and fuel tank capacity.
Why Generator Runtime Matters in Emergencies
During prolonged outages caused by hurricanes, blizzards, or grid failures, generator runtime becomes critical. Your ability to keep the generator running without constant refueling can determine:
- Whether your food stays cold
- Your home’s ability to stay heated or cooled
- How long sump pumps, security systems, or medical equipment function
- How much fuel you need to safely store in advance
The longer your generator can run on a single tank, the better prepared you’ll be to ride out a multi-day power outage.
Generator Runtime by Fuel Type
Let’s compare the average runtime for generators based on their fuel source. Actual runtime varies based on load percentage (how much power you’re using), generator efficiency, engine size, and tank capacity.
1. Gasoline Generator Runtime
- Typical fuel tank size: 3–8 gallons (portable generators)
- Runtime at 50% load: 8 to 12 hours
Gasoline-powered generators are commonly used in portable units. On average, they run 8–12 hours per tank at 50% load, which means they need to be refueled once or twice per day during continuous operation.
Example: A 7,500-watt gasoline generator with a 6.5-gallon tank may run 10–12 hours on half load before requiring more fuel.
2. Diesel Generator Runtime
- Typical fuel tank size: 5–20 gallons (larger in commercial units)
- Runtime at 50% load: 12 to 24+ hours
Diesel generators are known for their fuel efficiency. A diesel unit can easily run 12 to 24 hours on a full tank, and commercial models with larger tanks or external fuel connections can run even longer.
Example: A 12kW diesel standby generator with a 20-gallon tank may run continuously for over 24 hours depending on the load.
3. Propane Generator Runtime
- Typical tank size: 20 lbs (portable) to 100+ gallons (stationary)
- Runtime at 50% load:
- 20-lb tank: 4 to 6 hours
- 100-gallon tank: Up to 90 hours
Propane runtimes depend heavily on the size of the tank. Small 20-lb tanks (commonly used for grills) provide 4–6 hours of power at 50% load. However, homes with large stationary propane tanks can run a generator for several days without interruption.
Example: A 9kW standby generator connected to a 100-gallon propane tank may run up to 3.5 to 4 days at 50% load continuously.
4. Natural Gas Generator Runtime
- Fuel source: Connected directly to home’s natural gas utility line
- Runtime: Essentially unlimited as long as gas supply is active
Because natural gas generators are tied directly to a utility line, their runtime is only limited by gas supply. In theory, they can run indefinitely, making them the most convenient for long-term outages.
Important: In rare cases, natural gas may be shut off during disasters, such as earthquakes or major pipeline incidents, so always have a contingency plan.
Runtime Comparison Table (50% Load)
| Fuel Type | Typical Tank Size | Estimated Runtime | Best For |
|---|---|---|---|
| Gasoline | 3–8 gallons | 8–12 hours | Short outages, portable use |
| Diesel | 5–20 gallons | 12–24+ hours | Longer outages, heavy loads |
| Propane | 20 lbs – 100+ gal | 4 hrs – 90+ hrs | Clean power, storage-friendly |
| Natural Gas | Utility line | Unlimited | Whole-home, seamless backup |
Factors That Affect Generator Runtime
1. Load Level
The more power your appliances draw, the faster your generator will consume fuel. A generator running at 100% load may burn fuel twice as fast as one at 50% load.
2. Fuel Tank Size
Larger tanks offer longer runtimes. Some portable generators also support external tanks for extended operation.
3. Fuel Type
Diesel is generally the most fuel-efficient. Gasoline and propane have lower energy densities and will deplete faster under the same load.
4. Engine Efficiency
Newer inverter generators are more efficient and adjust output based on power demand. Older models or standard generators may burn more fuel even under light loads.
5. Weather Conditions
Extremely cold temperatures can reduce propane pressure, leading to decreased efficiency. Gasoline can also thicken and cause hard starts if not stabilized in advance.
How to Extend Generator Runtime
- Run only essential appliances: Prioritize refrigerators, sump pumps, and communication devices
- Cycle power use: Don’t run all appliances at once
- Use energy-efficient lighting and devices: Reduce overall load
- Perform regular maintenance: Clean air filters and check oil levels
- Store extra fuel safely: Use fuel stabilizers and approved containers
FAQs About Generator Runtime
Q: Can I run a generator overnight?
Yes, but it depends on the generator’s fuel tank size and runtime. Always place the generator outdoors, at least 20 feet away from your home, and use a carbon monoxide detector for safety.
Q: Can I refill the generator while it’s running?
No. Always turn off the generator and let it cool before refueling to prevent fire hazards or fuel spills.
Q: How much propane do I need to run a generator for 3 days?
At 50% load, a 9kW generator uses about 2–3 gallons of propane per hour. For 3 full days, you’ll need around 150–200 gallons. Always consult your manufacturer’s specs.
Q: Is there a generator that doesn’t require refueling?
Yes. Natural gas standby generators run off your home’s gas line and can operate continuously without manual refueling—ideal for long outages.
Top Generator Recommendations by Runtime
🔋 Longest Runtime Portable Gas Generator
Westinghouse WGen7500 – Up to 11 hours on a full tank at 50% load with electric start.
🔥 Best Dual Fuel Option (Propane/Gas)
Champion 7500-Watt Dual Fuel Generator – Offers 8–10 hours on gas or up to 6 hours on a 20-lb propane tank.
🏠 Best Long-Term Standby Generator
Generac Guardian 22kW – Connects to natural gas or large propane tanks for multi-day, automatic operation.
Conclusion: Plan Your Fuel Strategy Before the Storm Hits
Knowing how long your generator will run on a full tank of fuel or propane is key to surviving a power outage comfortably and safely. While gasoline generators offer quick and easy access, diesel and propane units are more efficient for longer outages. For the ultimate in convenience and extended runtime, natural gas standby generators offer hands-free peace of mind—if you have a gas line connection.
Plan ahead, stock up on safe fuel storage, and always follow safety and maintenance protocols to get the most from your generator when you need it most.
Still Unsure Which Generator Is Right for You?
Check out our full guide: What Size Generator Do I Need for My Home? for help matching generator size, fuel type, and features to your household’s emergency power needs.