Manufacturer runtime figures are always quoted at some fixed load, usually a quarter or half of rated output, and that is very rarely what you are actually running. A generator at eighty percent load burns considerably more than the box suggests.
The calculator below estimates burn from your real load rather than assuming a constant, and returns runtime per tank, gallons per hour, and cost. Our roundup of solar generators for home backup covers the fuel-free alternative.
Safety Warning
Never run a generator indoors, in a garage, or near open windows. Carbon monoxide from a generator is odorless and can be fatal within minutes. Outdoors and well away from the building is the only safe placement.
What Changes Fuel Burn
| Factor | Effect |
|---|---|
| Load percentage | The dominant variable by far |
| Rated output | Larger units burn more at the same load |
| Inverter versus conventional | Inverter units throttle engine speed |
| Fuel type | Propane burns less efficiently than gasoline |
| Altitude | Output drops at elevation |
| Engine condition | Maintenance affects consumption |
Load Percentage Is the Whole Story
A generator does not burn fuel at any fixed rate at all. It burns roughly in proportion to the work it is actually doing, plus a baseline amount just for keeping itself running.
That baseline is why a generator at ten percent load is inefficient rather than economical, since a substantial share of the fuel goes to keeping the engine turning.
At the other end, output climbs steeply above seventy percent, which is where the headline runtime figure diverges most from reality.
Knowing your actual load is therefore more useful than knowing the generator’s rating, and most people have never measured it.
Adding up nameplate watts on everything you intend to run is the starting point. Our note on what appliances you can run off grid covers building that list.
The Sweet Spot Is Not the Ceiling
Fifty to seventy five percent load is ideal
Efficient, and comfortable for the engine.
Above eighty sustained shortens life
Running near capacity continuously wears components faster.
Under twenty five percent is wasteful
Baseline consumption dominates at light loads.
Over one hundred percent is a fault
Either the breaker trips or something gets damaged.
Wet Stacking on Diesel Units
Diesel generators run persistently under light load develop a problem gasoline units do not.
Unburned fuel gradually accumulates in the exhaust system, which is where the term wet stacking actually comes from, and it fouls the injectors and reduces overall efficiency over time.
The fix is running the unit under meaningful load periodically rather than idling it, which burns off the accumulation.
That is why standby diesel installations are exercised under load rather than simply started monthly.
For a diesel unit that spends most of its time lightly loaded, a load bank exercise is worth building into the maintenance routine. Our note on maintaining a generator covers upkeep.
Inverter Generators Change the Numbers
A conventional generator runs its engine at a fixed speed regardless of load, because output frequency depends on it.
An inverter generator produces DC and converts it, which frees the engine to slow down when demand is low.
That produces substantially better fuel economy at light loads, and it is why inverter units advertise long runtimes at quarter load.
They are also quieter at low output for the same reason, which matters in a campsite or a neighborhood.
The trade is cost and typically lower maximum output for the money. Our roundup of power station versus generator covers the comparison.
Sizing for the Outage You Expect
Runtime per tank matters considerably less than the total fuel you have available and how long an outage you actually need to bridge.
A twenty four hour outage on a four gallon tank means refueling several times, which is fine if you are present and impossible if you are not.
Storing fuel introduces its own considerations, since gasoline degrades over months and needs stabilizer or rotation.
Propane does not degrade, which is much of why dual fuel units are popular for standby use despite burning slightly less efficiently.
Calculating total fuel for the outage length you actually plan for is more useful than optimizing a single tank. Our roundup of generators for RV and van life covers mobile setups.
Altitude and Temperature Reduce Output
A generator’s rating assumes conditions near sea level, and both elevation and heat reduce what it can actually deliver.
Thinner air at altitude means less oxygen for combustion, and manufacturers generally cite a percentage reduction in output for every thousand feet above a baseline.
High ambient temperature reduces output as well, since hot air is less dense and cooling becomes less effective.
That matters for load planning, because a generator rated comfortably above your load at sea level may sit uncomfortably close to it in a hot location at elevation.
Some units offer high altitude jet kits to restore part of the loss, and running without one at elevation also affects emissions and engine life. Our note on how many watt hours you need covers load planning.
Reducing the Load Rather Than Adding Fuel
The cheapest way to extend runtime is running less, and the savings are larger than most people expect given how burn scales with load.
Refrigeration is usually by far the largest continuous draw on a system, and cycling it rather than running it constantly reduces your average load quite meaningfully.
Resistive heating is the worst thing to put on a generator, since space heaters and electric water heaters draw enormous continuous power.
Staggering high-draw appliances rather than running them together keeps peak load down, which also protects against overload.
Anything that can run on propane directly rather than through the generator removes that load entirely. Our roundup of propane appliances covers those.
Before You Rely on It
Add up your actual running load
Nameplate watts on everything running simultaneously.
Check the surge rating quite separately
Motors draw several times running watts at startup.
Plan total fuel, not tank runtime
A long outage means refueling regardless of tank size.
Test it under real load before you need it
A generator that has not run in a year frequently will not start.
Common Mistakes to Avoid
Trusting the headline runtime
Those figures are quoted at a quarter or half load, not yours.
Running near capacity continuously
Above eighty percent sustained wears the unit faster and burns fuel steeply.
Idling a diesel unit under light load
Wet stacking fouls the exhaust and injectors over time.
Running it anywhere enclosed
Carbon monoxide is odorless and kills quickly. Outdoors only, away from windows.
Sources
US Consumer Product Safety Commission, on portable generator carbon monoxide hazards. Occupational Safety and Health Administration, on generator safety. US Department of Energy, on backup power systems. Environmental Protection Agency, on small engine emissions and fuel storage.
Recommended Reading
See our note on sizing a portable power station, our roundup of generator accessories, our note on battery monitors off grid solar, and a roundup of generators for CPAP machines.
Generator Runtime FAQ
Why is my runtime shorter than advertised?
Manufacturer figures are quoted at a fixed load, usually a quarter or half of rated output. Running closer to capacity burns considerably more.
What load should I aim for?
Fifty to seventy five percent of rated output. That range is efficient and comfortable for the engine. Above eighty sustained shortens its life.
Is running a generator lightly loaded better?
Not especially. Baseline consumption dominates at light loads, so it is inefficient. On diesel units it also causes wet stacking.
What is wet stacking?
Unburned fuel accumulating in a diesel generator’s exhaust from prolonged light-load running. It fouls injectors and reduces efficiency.
Are inverter generators more efficient?
At light loads, considerably. They let the engine slow down when demand is low, which a conventional generator cannot do.
Should I use gasoline or propane?
Propane does not degrade in storage, which suits standby use. Gasoline is slightly more efficient but needs stabilizer or rotation.
How do I extend runtime?
Reduce the load. Burn scales with load, so cycling refrigeration and avoiding resistive heating saves more fuel than most people expect.
Can I run it in the garage with the door open?
No. Carbon monoxide accumulates in enclosed and partly enclosed spaces and is fatal quickly. Outdoors, well away from doors and windows.
