How long a tank lasts, and why load decides it

A portable advertises ten hours on a tank. The first real outage empties it in five, and the owner assumes the machine is faulty.

It is not. The ten hour figure is true at the load the manufacturer quoted it at, which is usually a quarter or a half of capacity, and the house is asking for more than that.

Consumption has two parts

An engine burns some fuel simply to keep turning, before it does any useful work at all. On top of that sits a quantity proportional to the electrical load it is carrying.

That is why the relationship is not a straight line through zero. Halving the load does not halve the fuel. A 10 kW petrol set might burn around 0.9 gallons an hour at half load and still around 0.6 at a quarter load, because the fixed share does not shrink.

Working a real example

Take that 10 kW machine with a 5 gallon tank.

At half load, 0.9 gallons an hour gives about five and a half hours. At quarter load, 0.6 an hour gives over eight. Run it near full and you are into the low fours. Those are three different honest answers to “how long does a tank last”, and the only thing separating them is what you plugged in.

The generator fuel calculator takes the size, the fuel and the load share and returns consumption per hour and run time per tank, which is the pair worth knowing before an outage rather than during one.

Refuelling is the real constraint

Run time per tank matters because of what has to happen when it ends.

Most portables have to be shut down and allowed to cool before refuelling, since petrol onto a hot engine or exhaust is exactly the hazard it sounds like. So a five hour tank is not five hours of power, it is five hours followed by a cooling period in whatever weather is outside.

That single fact is why overnight running is awkward on a portable and why dual fuel machines on propane are popular. A propane cylinder can be swapped without the same cooling wait, and the fuel does not spill.

How much to keep on hand

Work it from the outage you are preparing for rather than from the tank.

A 48 hour outage at half load on that 10 kW machine is roughly 43 gallons of petrol, which is eight or nine standard cans. Most people discover this during the outage, which is also when the queue at the filling station is longest and the pumps are often down for the same reason your lights are.

Propane changes the shape of that problem. A 20 lb barbecue cylinder holds under 5 gallons and buys only a few hours on a machine this size, but 100 lb cylinders and larger tanks are practical, and the fuel keeps indefinitely, so a reserve bought once stays useful.

Smaller load, longer outage

The useful lever during a long outage is the load list rather than the fuel supply.

Dropping air conditioning or an electric range moves the machine from three quarters load to a half or a quarter, which stretches the same fuel considerably and makes refuelling intervals civilised. Knowing in advance which circuits you can shed, and what each one costs in kilowatts, is what makes that a decision rather than a scramble. The generator size calculator is the quickest way to build that list while the lights are still on.

A note on the exercise cycle

Standby generators run themselves briefly every week to keep the engine healthy, and on natural gas that is invisible because the fuel is piped.

On propane it is not. Those weekly runs draw from the tank all year, so a tank that was filled in spring and never used in anger can still be noticeably down by autumn. Checking the gauge before storm season is a two minute job that prevents the specific disappointment of a standby generator that starts, runs for twenty minutes and stops.