Generator Fuel Runtime: How Long Will It Run on Gas, Propane, or Diesel?
One of the most common backup-power questions is also the most misunderstood: “How long will my generator actually run?”
The answer depends on fuel type, generator size, and — most importantly — how much load you’re putting on it. This guide breaks it down without marketing fluff.
Jump to the tool: Fuel Runtime Calculator ↓
The #1 Rule of Generator Runtime
Generators burn fuel based on electrical load — not just engine size.
A lightly loaded generator can run 2–3× longer than the same unit near full output.
Manufacturer runtime claims are typically published at 25% load and 50% load. If you overload the generator (or run high-draw appliances), runtime drops fast.
Runtime by Fuel Type (Real-World Expectations)
Gasoline Generators
- Most common for portables
- Fuel can degrade over time (use stabilizer)
- Easy to refuel, but storage safety matters
Propane (LP) Generators
- Excellent long-term storage (doesn’t “go bad” like gas)
- Cleaner burn
- Lower energy density than gasoline → often shorter runtime per tank
Diesel Generators
- Often best efficiency per kWh
- Strong for heavier / sustained loads
- Common in standby and commercial setups
Dual-Fuel / Tri-Fuel Units
- Runtime depends on selected fuel
- Propane/natural gas improve fuel availability
- Output may drop on gaseous fuels (normal)
Fuel Runtime Comparison (Planning-Level)
| Fuel | Storage Example | Typical Runtime Range* | Best Use Case |
|---|---|---|---|
| Gasoline | 6–8 gal portable tank | ~8–14 hours | Short outages, portability |
| Propane | 20 lb cylinder | ~4–6 hours | Storage-friendly backup |
| Propane | 100 lb tank | ~1–1.5 days | Extended outages with clean fuel |
| Diesel | 50 gal tank | ~2–4 days | Heavy loads, long runtime |
Important
These ranges depend heavily on your load. Use the calculator below for a runtime estimate based on your watts.
🧮 Fuel Runtime Calculator
Enter your average load and fuel available. This estimates runtime using typical small-engine burn curves. It’s a planning tool — always verify with your generator’s manual if available.
Fuel Runtime Estimator (hours)
Strongest input: average load (watts). If you don’t know it, use your Load Calculator / Sizing Wizard first.
Estimated burn rate: — per hour
Runtime (hours)
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Runtime (hh:mm)
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How this estimate works
Runtime ≈ fuel available ÷ estimated burn rate. Burn rises with load.
Why Real-World Runtime Disappoints Homeowners
- Running electric heat, water heaters, or other huge resistive loads
- No load prioritization (everything on at once)
- Ignoring startup surge from pumps and compressors
- Underestimating cold-weather behavior and refuel logistics
The winning move is simple: control loads first. Then fuel planning becomes predictable instead of stressful.
Next Step: Lock Your Load List
Runtime math is only as good as your load estimate. If your watts are a guess, fix that first: