guide · energy
How Long Will a Generator Run on a Full Tank?
Updated September 29, 2026 · 6 min read
direct answer
Divide the tank size by gallons burned per hour. A 5 kW gasoline generator at 75% load burns 0.60 gallons an hour, so a 5-gallon tank lasts about 8 hours 20 minutes. The same tank gives 6 hours 15 minutes at full load and 12 hours 30 minutes at 50% — halving the load doubles the runtime.
The number printed on the box is a laboratory figure. This is the number your tank will actually do — and the rule that tells you when it is too small to trust overnight.
The formula, in one line
Runtime has no hidden variables:
- Runtime (hours) = tank size (gallons) ÷ fuel per hour (gal/h)
- Fuel per hour = kW drawn × SFC (0.16 gal/kW/h for gasoline)
Put the 5 kW example through it: 3.75 kW × 0.16 = 0.60 gal/h, then 5 ÷ 0.60 = 8.33 hours — 8 h 20 min. Two steps, and you can check any claim on any brochure in under a minute.
Runtime by tank size
The same 5 kW gasoline generator at 75% load (0.60 gal/h), across the tank sizes portable machines actually ship with:
| Tank size | Litres | Runtime at 75% load | Refuel stops per 24 h |
|---|---|---|---|
| 1 gallon | 3.8 L | 1 h 40 min | 14 |
| 2 gallons | 7.6 L | 3 h 20 min | 7 |
| 5 gallons | 18.9 L | 8 h 20 min | 2 |
| 10 gallons | 37.9 L | 16 h 40 min | 1 |
| 20 gallons | 75.7 L | 33 h 20 min | 0 |
Two rows in that table are not like the others. At 1 gallon the machine runs 1 hour 40 minutes — under two hours — which is where most portable inverter generators land. That is the number that decides whether you sleep through a storm or get up to refuel.
The 2-hour rule
Any generator whose full-tank runtime falls below about 2 hours is, practically speaking, a daytime tool. Under that threshold you are refuelling 12 or more times a day, in the dark, next to a hot exhaust, with a jerry can. It is not a safety lecture so much as an arithmetic one: nobody keeps that up for a full night.
Use it as a buying filter: if the calculated runtime on a full tank is under 2 hours, the tank is too small for unattended use. Fix it with a bigger tank, an external fuel line, a larger generator run at lower load — or by simply accepting that you will be awake with it.
Runtime by load: the lever that actually matters
Same 5 kW generator, same 5-gallon tank, four load settings:
| Load | Power drawn | Fuel per hour | Runtime on 5 gal |
|---|---|---|---|
| 25% | 1.25 kW | 0.20 gal | 25 h 0 min |
| 50% | 2.50 kW | 0.40 gal | 12 h 30 min |
| 75% | 3.75 kW | 0.60 gal | 8 h 20 min |
| 100% | 5.00 kW | 0.80 gal | 6 h 15 min |
Runtime is inversely proportional to load — cut the load in half, double the time. Which is why the cheapest upgrade for longer runtime is not a bigger tank, it is unplugging the water heater and letting the fridge cycle on its own. A fridge averages a few hundred watts; running it on a 5 kW generator puts you near the bottom of that table, at 20-plus hours on one fill.
Runtime by generator size
If you are choosing between sizes, here is how long a 5-gallon tank lasts at 75% load for the common portable range (0.12 gal/kW/h effective burn rate):
| Generator size | Fuel per hour | Runtime on 5 gal |
|---|---|---|
| 2000 W | 0.24 gal | 20 h 50 min |
| 3500 W | 0.42 gal | 11 h 54 min |
| 5000 W | 0.60 gal | 8 h 20 min |
| 7500 W | 0.90 gal | 5 h 33 min |
| 10000 W | 1.20 gal | 4 h 10 min |
Bigger engines are not thirstier for the sake of it — they are supplying more power, and fuel use tracks power output almost exactly. The efficiency win comes from not buying more generator than you need.
Five ways to stretch a tank
- Run eco-mode. Most inverter generators drop to 50% or less of rated load for steady work, roughly doubling runtime.
- Shed the phantom loads. Water heaters, AC units and electric kettles are what push you from the 50% row to the 100% row. If you are not sure which appliances deserve to stay on, the ranked appliance cost table shows what each one draws in normal use.
- Keep the air filter clean. A clogged filter richens the mixture and costs fuel, power and engine life at the same time.
- Do not let the tank run dry. Beyond the obvious stop, an empty tank sucks debris into the carburettor — the single most common way portable generators die.
- Store fuel properly. Fresh gasoline, or fuel stabiliser if the machine will sit. Old fuel causes hard starts that make people think the generator failed.
try it with your tank
Generator Fuel Runtime & Cost Estimator
Enter your generator size, fuel type, tank size and load. You get fuel per hour, full-tank runtime, cost to fill and cost per hour — and a red warning when runtime drops under 2 hours.
Open the calculatorfaq
Frequently Asked Questions
Tank sizing, overnight runtime and the tweaks that buy you extra hours.
How long will a generator run on a full tank?
Divide the tank size by fuel consumed per hour. A 5 kW gasoline generator at 75% load burns 0.60 gallons per hour, so a 5-gallon tank lasts about 8 hours 20 minutes. The same tank lasts 6 hours 15 minutes at full load and 12 hours 30 minutes at 50% load.
What size fuel tank do I need for overnight power?
For an unattended 8-hour night on a 5 kW gasoline generator, plan on at least 5 gallons (19 litres) — the point where runtime reaches about 8 hours at 75% load. Below 2 gallons the machine needs refuelling every 1 to 3 hours, which makes it impractical for overnight use.
How do I make my generator run longer on a tank?
Drop the load: cutting from 100% to 50% roughly doubles runtime, from 6.25 to 12.5 hours on a 5-gallon tank. Remove unloaded appliances, use eco-mode, keep the air filter clean, and never let the tank sit below a quarter — stale, empty tanks cause the carburettor problems that end generator lives.