Load sets the answer, not the tank
Two numbers decide run time: how much fuel you are carrying, and how fast the engine drinks it. Only the second one moves. A 7,500 W generator with a 6.6 gallon tank runs just under ten hours at half load, about 21 hours carrying 1,000 W, and under six hours flat out. Same tank, same fuel, three very different answers.
That is why a run-time claim on a box is close to meaningless on its own. Manufacturers quote their figures at 25% or 50% of rated load with one steady appliance attached. Your load is lumpy, and it is almost never the load they tested. If you want the number for your own equipment list, the power sizing calculator works it out from what you actually plug in.
The fuel model behind every number here
There is no fuel gauge in a spreadsheet, so this site uses one small model for every generator figure it prints. An engine burns roughly 0.13 gallons of gasoline per kilowatt-hour it delivers, plus a small idle burn of about 0.02 gallons per hour for each kilowatt of engine rating just to keep itself spinning. Conventional, non-inverter engines get 0.14 gallons per kWh and a 0.025 idle figure, which makes them about 15% thirstier.
That model was fitted to Honda's published EU2200i run times: a 0.95 gallon tank, 8.1 hours at 25% load and 3.4 hours at the rated 1,800 W. Working the same machine at half load, 900 W, gives 0.036 gallons an hour of idle burn plus 0.117 for the work, so 0.15 gallons an hour and about 6 hours 15 minutes on a tank.
Be clear about what that is worth. It is a model, not a measurement of your engine. Against a real machine on a real day, expect a modelled fuel figure to be good to roughly plus or minus 20%. Altitude, a tired carburetor, stale fuel, ambient temperature and a load full of motors all push it around. Use it to choose a tank size and plan a fuel run, not to bet the last half hour of an outage on it.
Run time at half load, by generator size
This is the same 50% load point manufacturers quote against, computed for every generator class in the calculator.
| Generator | Tank | At half load | Burn | Run time |
|---|---|---|---|---|
| 1,000 W inverter | 0.60 gal | 450 W | 0.08 gal/h | 7 h 50 min |
| 2,200 W inverter | 0.95 gal | 900 W | 0.15 gal/h | 6 h 15 min |
| 3,500 W inverter | 1.8 gal | 1,500 W | 0.26 gal/h | 7 h 5 min |
| 5,000 W | 4.0 gal | 2,250 W | 0.43 gal/h | 9 h 20 min |
| 7,500 W | 6.6 gal | 3,500 W | 0.67 gal/h | 9 h 55 min |
| 9,500 W | 6.6 gal | 4,250 W | 0.81 gal/h | 8 h 10 min |
| 12,000 W | 8.3 gal | 5,500 W | 1.0 gal/h | 7 h 55 min |
| 15,000 W | 10 gal | 6,000 W | 1.1 gal/h | 8 h 45 min |
Look down the last column. Almost every class lands between six and ten hours, because makers size the tank to get roughly a working day out of the machine. Buying a bigger generator does not buy you a longer run. It buys you more watts, and it usually costs you fuel.
A real load runs far longer than half load
Half load is a test condition. Here is a real one, taken from the home backup list in the calculator: a refrigerator, a chest freezer, a furnace blower, a sump pump, LED lighting, a router, a TV and two phones.
Add up the running watts and you get 2,100 W. But those items cycle. The fridge compressor is on about a third of the time, the sump pump runs in short bursts, the furnace blower comes and goes. The duty-weighted average is only 670 W.
On a 7,500 W generator that average works out at about 0.27 gallons an hour, so 6.6 gallons lasts roughly 24 and a half hours and a full day costs about 6.4 gallons. If every one of those items somehow ran flat out the whole time, you would get 14 hours instead. The honest planning number sits between the two, closer to the first.
Notice what makes up that 0.27 gallons an hour: 0.175 of it is idle burn from a 7 kW engine loafing along under a 670 W load. Nearly two thirds of the fuel is going into spinning the engine, not into your refrigerator.
Why inverter run times look so good
An inverter generator throttles its engine down to match the load and rebuilds clean AC electronically. A conventional generator has to hold a fixed engine speed to produce 60 Hz, loaded or not. The gap shows up at light loads and it is large.
Deliver 500 W. A 2,200 W inverter burns about 0.10 gallons an hour. A 7,500 W conventional unit doing exactly the same work burns about 0.25. Put that home backup load on a 3,500 W inverter instead of the 7,500 W conventional machine and the day's fuel drops from 6.4 gallons to about 3.5, for the same fridge and the same furnace.
Propane, and what a 20 lb tank really gives you
Propane holds less energy per gallon than gasoline, about 91,500 BTU against 114,000. The practical effect: the same generator burns about 1.25 gallons of propane for every gallon of gasoline, so a gallon of propane buys you about 20% fewer hours.
| Container | Fuel | Holds |
|---|---|---|
| 20 lb propane tank | Propane | 4.7 gal |
| 30 lb propane tank | Propane | 7.1 gal |
| 100 lb propane tank | Propane | 24 gal |
| 1 gallon tank | Gasoline | 1.0 gal |
| 5 gallon can | Gasoline | 5.0 gal |
| 8 gallon tank | Gasoline | 8.0 gal |
A 20 lb barbecue tank holds about 4.7 gallons. On a 7,500 W generator at half load that is roughly 5 hours 40 minutes, where the same 4.7 gallons of gasoline would give about 7 hours. On a 2,200 W inverter at half load, one 20 lb tank runs about 24 hours. Propane pays you back in other ways: it does not go stale in storage, and it starts cleanly after a year in the shed.
What a day of running actually costs
Continuous running is a fuel-logistics problem before it is a power problem. A 7,500 W unit held at half load wants about 16 gallons a day, which is three 5 gallon cans and change. A 12,000 W unit at half load wants about 25. Real outage loads are far gentler: that home backup list needs around 6.4 gallons a day on the same 7,500 W machine.
Two practical notes. Refuel a cool engine, never a hot running one. And check oil daily, because oil level, not fuel, is what kills generators left running for days.
Leave 20% of the rating alone
Size so your continuous load sits at or below 80% of the generator's continuous rating. On a 7,000 W running rating, that is 5,600 W. The calculator applies it as a 1.25 multiplier when it picks a size for you.
The reason is heat and headroom. An engine held near its ceiling for hours runs hot, drops voltage when a compressor kicks in, and has nothing left for the surge. Starting watts are a separate problem from running watts, and the difference between starting and running watts is what decides whether a generator that easily carries your load can still stall trying to start it. In the home backup example above, the running total is 2,100 W but the peak is 3,700 W.
If none of these fuel numbers appeal, the trade against stored power is covered in generator versus portable power station. Otherwise, put your own equipment into the calculator and read the burn rate off your real average load rather than a brochure's.