Generator Run Time Table
| Unit | Fuel | 25% load | 50% load |
|---|---|---|---|
| 2000W inverter | ~1 gal tank | ~8-9 h | ~4-5 h |
| 3500W portable | 2.6 gal | ~11 h | ~7 h |
| 4500W inverter | 3.4 gal | ~14 h | ~9 h |
| 7500W portable | 7 gal | ~15 h | ~10 h |
| 10-22kW standby | 500 gal propane | days - 30-60 gal/day at house duty | |
| Outage day (typical duty) | Avg load | Gasoline/day | Propane/day |
|---|---|---|---|
| Fridge, phones, lamps | ~300 W | ~1 gal | ~1.3 gal |
| + furnace fan, sump | ~900 W | ~3 gal | ~3.5 gal |
| + well pump, some cooking | ~1800 W | ~6 gal | ~7 gal |
Run time is a load story, not a tank story: the same 3500W portable that hums 11 hours on 2.6 gallons at quarter load drops near 7 at half load, and an open-frame unit runs roughly twice the fuel of an inverter at the same light load because it cannot throttle down. Manufacturers quote hours at 25 or 50 percent load in eco mode - honest numbers, but optimistic for the outage you are imagining, because a real house cycles a fridge, a furnace fan and lights in bursts rather than sitting at a steady fraction.
Bottom line: budget about 0.08 to 0.12 gallons of gasoline per kilowatt-hour actually delivered, or roughly a tank a day for a portable doing house duty - a 2000W inverter on a gallon-class tank covers a fridge, phones and lamps overnight at quarter load, while a 7500W unit on a 7-gallon tank is a two-refuel-a-day machine at half load. Whole-home standby changes the units entirely: on a 250 to 500 gallon propane tank, a 10 to 22kW unit sips for days, which is the entire argument for the standby premium.
The honest part: propane costs about 10 to 15 percent of its run time versus gasoline on the same unit (lower energy density), dual-fuel figures always look best on the gasoline side, and every manufacturer number assumes fresh fuel, a broken-in engine and eco mode engaged. Refueling a hot generator is the house-fire move - the runtime column is also a maintenance schedule: portable units want an oil check at every fill and a cool-down pause, and any runtime math is void the moment the unit is running inside a garage.
How to use
- Find your size row in the table and read the load column closest to what you will actually run - most outage duty lives between quarter and half load.
- Convert to your outage: average load in watts times hours per day gives the fuel per day, and refuels per day is the number that decides whether the setup is livable.
- Stretch hours with load discipline, not a bigger tank: cycle the fridge, run the well pump in scheduled blocks, keep the inverter on eco, and refuel only cool.
Frequently asked questions
How long will a generator run on a tank of gas?
By the table: a 2000W inverter on its ~1 gallon tank runs about 8 to 9 hours at quarter load, 4 to 5 at half; a 3500W portable on 2.6 gallons runs near 11 at quarter and 7 at half; a 7500W class unit on 7 gallons runs about 10 at half load. Divide tank gallons by the per-hour burn for your load and you have the honest number - manufacturers websites quote the same curve, at the flattering end.
How long can you run a generator continuously?
Portables: all day with discipline - an oil check and cool-down at each refuel, and most manufacturers cap continuous duty around 8 to 12 hours before a rest. Standby units are built for multi-day runs on natural gas or a big propane tank with scheduled maintenance windows. The universal rules: never indoors or in a garage (carbon monoxide kills quietly), 20 feet from the house, and refuel only cold.
How much fuel does a generator use per day?
A light-duty inverter day (fridge, phones, lamps, ~300W average) is about 1 gallon; a house-duty portable day (fridge, furnace fan, sump, some lights, ~1 to 1.5kW average) is 3 to 6 gallons of gasoline; a 10kW-plus standby at similar duty is 30 to 60 gallons of propane a day - which is why the 500-gallon tank buys roughly a week of normal life and why load management is the cheapest fuel there is.
Does eco mode really save fuel?
Yes, and it is the biggest free runtime there is on an inverter: the engine throttles to the load instead of screaming at 3600 RPM all day, cutting burn by 20 to 40 percent at light loads and the noise with it. Open-frame portables mostly lack it, which is exactly why the inverter row of the table wins the overnight-shift duty - same outlets, half the fuel, a quarter of the racket.
Is propane or gasoline better for run time?
Gasoline wins runtime per gallon by roughly 10 to 15 percent and stores badly beyond a season without stabilizer; propane stores indefinitely, burns cleaner in the carburetor, and prices per gallon of duty roughly even. For an outage kit the practical answer is dual-fuel: gasoline for the hours you planned, propane for the weeks you did not, since the 20-pound grill tank in the garage is emergency fuel you already own.
What size generator runs a fridge and furnace?
A 2000W inverter handles it comfortably: a fridge averages 100 to 200 running watts with a 600 to 900 start surge, a furnace fan 400 to 800, and the inverter's surge headroom absorbs the starts. Add a sump pump or a well pump and you are at the edge of 2000W and comfortably inside 3000 to 3500W - the size-calculator's job, with this table answering how long the fuel lasts once sized.