Generator fuel cost is one of the most under-planned parts of an extended outage — the generator itself is a one-time purchase, but a multi-day outage can mean a genuinely significant, ongoing fuel bill.
Rough daily cost by generator size
| Generator size | Typical fuel burn (50% load) | Approx. 24-hour cost (gasoline) |
|---|---|---|
| Small (2,000-3,500W) | ~0.2-0.3 gal/hr | ~$15-25 |
| Mid-size (4,000-6,500W) | ~0.4-0.6 gal/hr | ~$30-45 |
| Large (8,000-13,000W) | ~0.7-1.0 gal/hr | ~$50-75 |
These are rough planning figures at moderate (50%) load and typical U.S. gasoline prices — your generator's actual fuel-burn spec sheet (usually listed at 25%, 50% and 100% load) gives you a more precise number for your specific model. Running a generator at a lower load than its rated capacity generally improves fuel efficiency somewhat, which is part of why correct sizing (see our generator sizing guide) matters for cost, not just capability.
Propane and natural gas cost differently
Propane generally costs more per running hour than gasoline for the same generator, while natural gas (piped, not tank-limited) is often the cheapest option where available — see our natural gas vs propane comparison for the full tradeoff, since cost isn't the only factor.
How this stacks up over a multi-day outage
A 3-5 day outage running a mid-size generator at moderate load can easily mean $150-250+ in fuel alone — a real number worth budgeting for, and one of the arguments in favor of a correctly-sized (not oversized) generator, plus running only the essential circuits rather than everything at once. See our 5-year cost comparison for how this compares to a battery-based power station's near-zero running cost.