⏱️ Runtime Estimator
Fuel consumption scales with load. Running at 50% draws roughly half the fuel of 100% load.
Understanding Generator Fuel Consumption
Manufacturer runtime specs are virtually always published at 50% load. When you run a generator at 75–100% load you'll see significantly shorter runtime. Here's a general reference for common generator sizes:
| Generator Size | At 25% Load | At 50% Load | At 100% Load |
|---|---|---|---|
| 3,500W (3.5 gal tank) | ~13 hrs | ~9 hrs | ~5 hrs |
| 5,500W (6 gal tank) | ~16 hrs | ~10 hrs | ~6 hrs |
| 7,500W (6.6 gal tank) | ~14 hrs | ~9 hrs | ~5.5 hrs |
| 10,000W (10 gal tank) | ~16 hrs | ~10 hrs | ~6 hrs |
Why Load Percentage Matters
Generators use a throttle-governed engine — the engine works harder as load increases, burning more fuel per hour. The relationship isn't perfectly linear; at very light loads (under 30%) some generators actually become less efficient per watt produced. The sweet spot for most portable generators is 50–75% of rated output.
Outage planning tip: Budget for 2–3 tank refills per day for a multi-day storm outage if running at moderate load, especially if using central A/C. A 7,500W generator running at 65% load burns about 0.85 gal/hr — roughly 20 gallons per day. Pre-fill several approved gasoline containers before a storm arrives.
Propane vs. Gasoline Runtime
Dual-fuel generators can run on propane, which stores indefinitely (gasoline degrades in 30–90 days). However, propane is roughly 10–15% less efficient than gasoline — a generator rated at 9 hrs on gas will get approximately 7.5–8 hrs on propane. Factor this into your fuel stockpile calculations.
Frequently Asked Questions
It depends entirely on generator size and load. A 3,500W generator running moderate load (50%) burns about 0.5 gal/hr and gets ~10 hours from 5 gallons. A 7,500W generator at moderate load burns ~0.75–0.85 gal/hr for ~6–7 hours. Use the calculator above with your specific values.
No — this is a serious fire risk. Engine components get extremely hot, and gasoline vapors can ignite. Always turn off the generator, let it cool for at least 10–15 minutes, then refuel away from structures. Never refuel near open flames or while smoking.
Divide your total running watts by the generator's rated watts. Example: 4,200W on a 7,500W generator = 56% load. Use our Generator Size Calculator to find your total running watts. For the most accurate reading, some generators have a built-in load meter or you can use a clamp meter on the output circuit.
Yes, but with diminishing returns. Going from 100% to 50% load roughly halves fuel consumption. Going from 50% to 25% saves noticeably less because engine mechanical losses become a higher proportion of fuel burn. Running below 30% for extended periods can also cause "wet stacking" in diesel generators (raw fuel buildup in exhaust) — this is less of a concern on gasoline units.