How Long Will a Generator Run on X Gallons?
Estimate generator runtime based on fuel amount and consumption rate, with real examples and tools.
Quick answer
A generator consuming 0.75 gal/hr on 10 gallons will run for roughly 13 hours. The only variables are how much fuel you have and how fast your generator burns it.
How to find your generator's consumption rate
Every generator spec sheet lists fuel consumption at 50% load and often at full load. These are the most useful numbers for planning because generators rarely run at 100% capacity in real-world home backup use. If you have the manual, look for "fuel consumption" or "run time" specs. If you don't, the table below gives realistic ballpark rates by generator size.
Typical fuel consumption by generator size
| Generator size | At 50% load (gal/hr) | At full load (gal/hr) | Common use case |
|---|---|---|---|
| 2,000W (inverter) | ~0.10 | ~0.20 | Camping, small devices |
| 3,500W (inverter) | ~0.20 | ~0.35 | Essentials, fridge, lights |
| 5,500W (conventional) | ~0.50 | ~0.85 | Home partial backup |
| 7,500W (conventional) | ~0.75 | ~1.20 | Home backup, sump pump |
| 9,000–10,000W | ~1.00 | ~1.50 | Larger home backup |
| 12,000W+ | ~1.30 | ~2.00+ | Whole-home or standby |
Run your specific generator and load through the Generator Fuel Runtime Calculator.
These are approximations. Inverter generators (like the Honda EU2200i or Yamaha EF2200iS) are significantly more efficient at partial load than conventional generators of the same rated wattage because they vary engine speed with demand.
Runtime table, hours of run time by fuel amount
| Fuel available | 0.20 gal/hr | 0.50 gal/hr | 0.75 gal/hr | 1.00 gal/hr | 1.25 gal/hr |
|---|---|---|---|---|---|
| 5 gallons | 25h | 10h | 6.7h | 5h | 4h |
| 10 gallons | 50h | 20h | 13.3h | 10h | 8h |
| 15 gallons | 75h | 30h | 20h | 15h | 12h |
| 20 gallons | 100h | 40h | 26.7h | 20h | 16h |
| 30 gallons | 150h | 60h | 40h | 30h | 24h |
| 50 gallons | 250h | 100h | 66.7h | 50h | 40h |
Why runtime varies in practice
- Load level is the biggest factor: Consumption scales almost directly with load. A generator running at 25% capacity burns far less than one at 75%. In a home backup scenario where you're running essentials only (fridge, lights, phone charging, a fan), most generators operate well below 50% rated capacity, meaning real-world runtime is often longer than spec sheet numbers suggest.
- Inverter generators throttle down at low load: A conventional generator runs the engine at a fixed 3,600 RPM regardless of how much load is on it. An inverter generator reduces engine speed as load drops, which dramatically cuts fuel consumption. For modest loads, an inverter generator can run 2–3× longer than a comparably sized conventional on the same fuel.
- Cold weather reduces efficiency: Cold air is denser and may require the engine to run richer, increasing fuel consumption slightly. More significantly, in extreme cold, a carburetor generator may struggle to start or run smoothly without warmup time.
- Startup surges: Motor-driven appliances (fridges, sump pumps, well pumps, AC units) draw 2–3× their running wattage for a fraction of a second on startup. This doesn't meaningfully affect fuel consumption over time, but it affects what the generator can safely handle simultaneously.
- Fuel quality and age: Old, degraded gasoline reduces engine efficiency. Ethanol-blended gas (E10) has slightly less energy per gallon than ethanol-free fuel. Adding fresh fuel and using stabilizer for stored fuel helps maintain rated consumption rates.
Planning a multi-day fuel supply
For a realistic 7-day emergency scenario, here's how to think about fuel quantity:
- Assume 8 hours/day of generator run time (running essentials during the day, off at night)
- For a 7,500W conventional at 50% load: 0.75 gal/hr × 8 hrs/day = 6 gallons/day
- 7 days × 6 gallons = 42 gallons needed for a full week
- Most households legally store 25 gallons at home, supplemented with a trip to a gas station at the start of an outage event
- Reducing runtime to 6 hours/day or running a smaller, more efficient load drops that to 25–30 gallons for the same week
The practical takeaway: fuel efficiency matters enormously for extended outages. Right-sizing your generator to your actual load, not buying a 10,000W unit when 5,500W covers your essentials, is one of the best ways to extend your fuel supply.
Simple takeaway
Divide your available fuel by your generator's consumption rate to get runtime. At 50% load, a 7,500W conventional burns roughly 0.75 gal/hr; a 3,500W inverter burns closer to 0.20–0.25 gal/hr. Plan a safety buffer of at least 20%; consumption estimates are approximations, and you don't want to run dry mid-emergency.