Can You Refuel a Portable Generator While It’s Running?

When the power goes out you get to save the day by busting out your trusty generator and restore relative comfort to your family inside your house. After a few hours, you realize you probably should add some more fuel so that the electronics aren’t damaged if the generator starts starving for gas.

Since everyone’s watching a movie or playing a game at the table, you don’t want to plunge them back into stoneage darkness as you add some more gasoline to the tank. You unscrew the gas cap, pick up one of your 30lb modern red gas cans, fumble with the safety mechanism, put too much torque on the nozzle as you start to pour and realize that gas is leaking all over the scalding hot exhaust…

Yes, you just made a critical error in judgement and it could lead to damaging the generator, burning yourself, creating an explosion, or worse.

As a general rule, it is never safe to refuel your portable generator while it is running. Generator gas tanks are typically located right above or beside the extremely hot exhaust and this can lead to the unsafe ignition of spilled gas or concentrated vapors. It is best to first turn off your appliances, then shut off your generator and allow it to cool for 10 minutes before refueling.

Why Refueling a Running Generator is Dangerous

As you saw in the answer above, a portable generator is a very condensed unit with an engine attached to the power producing generator. It isn’t spaced out like a car where the engine is in the front and the gas tank is underneath in the rear of the vehicle.

Generators typically have their gas tanks on the very top of the unit. This allows many models to be gravity fed. Gasoline will naturally run “downhill” through the fuel lines at the bottom of the tank and make its way to the carburetor bowl.

Some models also have diaphragm fuel pumps that operate from positive and negative pressures of the crankcase.

Either way, those models still generally have the fuel tank on top.

If the gasoline tank is on the top, then naturally the exhaust will be located somewhere beneath it or directly adjacent to it.

If you touch the exhaust while the unit is in storage, it’s obviously not a problem. If you touch it after 60 seconds of the engine running, you’re going to be uncomfortable for that split second. If you touch it after a few minutes of running, you’re going to end up using burn cream and hearing sizzling flesh. Trust me, I’ve done it twice on my inner-forearm when lifting lawnmowers in the back of a truck — yes, I’m a slow learner.

The danger with refueling a generator when its running isn’t that you’re worried about burning yourself directly (though that can certainly be a byproduct of what’s next), it’s that you risk spilling liquid gasoline on an exhaust that will generally be between 600 and 1,000 degrees Fahrenheit (315-537 Celsius).

Gasoline has an auto-ignition temperature of 536 degrees Fahrenheit (280 Celsius).

That means that if you spill it on a hot exhaust, it will be igniting! That trail of fire could flow right up to your gas can that you’re leaning over, run down your legs from the gas that you spilled, or simply create a fire all around your generator. Either way, it’s not my definition of fun when the power is out, or any other time.

Even if you received your merit badge for gasoline pouring, you still have to be cautious about the vapors. Those pesky vapors are what create the most “bang”, so to speak. The liquid catches on fire, the vapors are what combust and do the work in your engine.

It might be a bit more rare for it to occur, but you can’t count out the vapors or fumes when it comes to the dangers. If you’ve worked around small engines enough, you have certainly seen exhausts that have screaming hot pieces of carbon come flying out or get stuck on the spark arrestor. You might also experience an “afterfire”, where flame comes out of the exhaust due to an excess of fuel that doesn’t fully combust and decides to ignite in the exhaust itself.

If those ignition sources encounter enough concentrated vapor, it probably won’t end well.

Again, none of these scenarios are my idea of fun and during a power outage with an inconvenienced family, I don’t want to be adding a date with the county EMS to my plate.

How Long to Let a Generator Cool Before Refueling

As a general rule, it is best to allow your generator and the hot exhaust to cool for 10-15 minutes before refueling. This is the generally agreed upon, minimum time to allow your hot exhaust to cool so that it won’t cause auto-ignition of any spilled gasoline.

In practice, I’ll admit that I have refueled mine after letting it cool for 5 minutes, and I’m assuming I’m not alone in that endeavor. I’m guilty and ashamed, and perhaps more lucky than anything. I wouldn’t recommend anyone else do it and I believe 10 minutes is a very acceptable number to live by.

However, I do have one cool thing I think you’ll be interested in that will make the process a lot safer and I’ll cover that next.

What is the Safest Way to Refill a Running Generator That is Low on Fuel

The safest way to refuel your generator after it has been running is to do the following in order:

  1. Turn off your appliances that are being powered
  2. Turn off the power on your generator (the circuit breaker, not the engine)
  3. Let the engine rev down for 60 seconds to prevent afterfire (flame through the exhaust after a heavy load and premature shutoff)
  4. Turn off the engine
  5. Allow the generator (exhaust) to cool off for 10-15 minutes
  6. Carefully refill the gas tank

How to Keep Gasoline from Spilling When Refueling a Generator

If you’ve bought a red gas can since 2009, you know just how fun and easy to operate they can be! In order to protect the environment from evaporative hydrocarbons and spilled gasoline by children or adults who aren’t paying attention, we mandated that all new gas cans must be almost perfectly sealed and have a two handed safety device on the spout that is simple to operate as you use your other two hands to lift the 35lbs gas can.

Catching my sarcasm yet?

In all fairness, I love how the new gas cans create a perfect seal (or as close to perfect as you can practically get). This makes long-term storage amazing (keeps oxygen out and prevents the volatile and most combustible “light ends” in the vapors from evaporating) when you combine the right amount of SeaFoam in your fuel. I have kept gas up to 2 years and it functioned flawlessly.

Other than that though, the amount of evaporative hydrocarbons that are saved by the perfect seal are negated by the amount of gas that you inevitably spill when trying to fill a gas tank.

Not only do you have to lift the 5 gallons up (31.5 pounds or 14.28kg) with two hands, you also need your two hands to toggle the safety feature where to twist something and press at the same time. All the while you are putting torque on the collar where the spout meets the gas can and gas always leaks by the rubber seal that is now being stressed because you can’t hold the gas can on your thigh and work the safety feature simultaneously.

Rant over.

Some people revert their gas cans by drilling a vent hole and installing an aftermarket spout that is reminiscent of the golden age of gas cans where you simply pop the vent, direct the nozzle, and pour. Gas didn’t last as long in storage, but you weren’t spilling it all over the exhaust and your pants at the same time.

I prefer a method that keeps that original spout intact so that I can store gas for a long time, but incorporates an inexpensive tool that allows for precision pouring without the spills.

If you haven’t bought one of these pumps from Amazon, I can’t adequately explain what you’re missing.

This tool will allow you to simply unscrew the collar around the spout of your gas can and remove, and then put one end down to the bottom of your gas can. Put the hose in your generator’s fuel tank, and press the button. It’s literally that easy.

battery operated fuel pump
This battery-operated fuel pump is worth EVERY PENNY!

No spills, and it can pump 5 gallons in 3 minutes. Set the gas can on a chair if it needs to be lifted higher and you will be saving your back and your children’s ears from your pirate lexicon from spilling gas all over your shoes.

Better yet, this pump makes it super easy for draining any gas tanks of fuel that you know should be replaced. If you have fuel that’s getting a bit old in your generator during storage, simply pump it out and back into a gas can. Then pump that gas into your car for immediate usage.

This is money well spent and is one of my favorite tools in my garage.

What Happens if You Let a Generator Run Out of Gas?

I just knew that if you made it this far that you’d find yourself asking this question.

When a generator runs out of gas, it will cause wild fluctuations in the supplied voltage and hertz to your appliances and electronics which can easily damage or destroy them. It is best to properly refill your generators gas tank before it runs out.

When a generator gets to the end of its fuel supply, it will start to “hunt”, which is when you hear the engine surging as its trying to suck any remaining gas in the fuel lines and the fuel at the bottom of the carburetor bowl into the combustion chamber.

Mechanically, nothing is going to be damaged with the engine of your generator when it runs out of gas. It’ll simply stall after its 30-second, grunting temper-tantrum.

However, your engine speed is what dictates the power that is produced. When you have wildly surging engine RPM’s, you highly risk damaging any appliances or electronics that are plugged into your generator.

Your devices and appliances are designed to run on clean electricity from your wall outlets. Fluctuations are extremely rare on a day to day or even monthly basis with the power supply. They need 120v and 60hz, for example.

An engine that is hunting for gasoline might start wildly fluctuating its voltage back and forth from 100v to 140v and 50hz to 70hz in those last 30 seconds. That may very well be enough to fry anything that you have plugged into it.

I highly recommend consulting with your owners manual (many can be found online) and taking note of how long they estimate a full tank to last at different loads. Then, write that on a piece of tape and stick it on the gas tank somewhere.

When you need to use your generator, make sure it’s filled up, take note of that piece of tape and set a timer on your phone for 3/4 of that time. When the timer goes off, follow the 6 steps above in the middle of this article.

If you do this, you’ll make sure that you never damage your electronics and appliances by letting your generator run out of gas.

Robert Van Nuck

Robert lives in central Michigan and enjoys running, woodworking, and fixing up small engines.

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