Gas Up The Generator? It’s Not That Simple.

If you are an off grid ham, you probably have a gas powered generator. And if you are a conscientious off grid ham, you have at least a few day’s worth of fuel stored. Does gas have a shelf life? Are all types of fuel the same? Are you unknowingly damaging your equipment by using improper fuel or improperly stored fuel? Have you ever even thought about the importance of fuel selection, storage, and handling? Understanding the gas that goes in your generator and getting the most out of it is a complex balance of chemistry, fuel management, and parsing myths and facts.

Keeping it fresh.

To answer the first question, yes, gasoline absolutely does have a shelf life. Over time the chemical components will begin to separate out and turn into a varnish-y slime. And for gas with alcohol added, which these days is pretty much the standard, this is particularly a problem because water is attracted to alcohol (more about this later). Very few of us use our generators every day. That means your reserve supply will very likely go stale before it is used.

The most straightforward way to avoid “stale gas” is to rotate your supply. Gas additives like Sta-Bil and Sea Foam will extend the shelf life of gas, up to a point. These products combine with the components in the gas and will retard –but not completely halt– the chemical breakdown process. Use them because they are effective, but don’t expect it to be a magic pill.

How you store your gas matters a lot. Gas cans should be kept 95% full, be tightly sealed, and stored in a dry place. The idea is to keep moisture out. Air carries water with it, so maintaining a minimum air volume inside the container reduces the opportunity for water to condense and accumulate. This is also why you should keep your generator fuel tank nearly full. Old school metal gas cans are still available and are better suited for fuel storage than the more common and less expensive plastic jugs.

The best gas can is…your car?

Another way to keep gas current is to use your car as the “storage container”. Since most people drive their car a lot more than they run a generator, your stock will rotate on its own so there are no concerns about “stale” fuel.



If you want to pull gas out of your car’s tank to refill the generator be aware that most cars have an anti-siphon/spill device in the filler pipe. I’m not going to drift into a detailed how-to, but there are tons of YouTube videos and internet resources out there that will guide you around this problem. There are also devices on the market that will defeat the anti-siphon/spill valve and let you remove gas from your car. These cool gizmos come in both electric and gravity feed versions and are an excellent accessory for the off grid ham. You can literally turn your car into a rolling mini-gas station.

If you think of it in terms of not having to keep several gas cans sitting around, not having to buy gas preservatives, plus not constantly having to deal with rotating gas (and the related expenses), transfer pumps/siphons are an investment that pays for itself. My vehicles can hold nearly fifty gallons of gas between them. That’s way more than I could realistically store in individual cans and the rotation issue takes care of itself. I can simply fill a small 2 gallon can as needed or pump gas from my vehicle directly to the generator.

Going through the corn maze.

For many decades, gas was gas. It didn’t require a lot of thought. You just fill ‘er up and go. Today there is pure gas, 10% ethanol, 85% ethanol, and soon-upcoming 15% ethanol. What is all this, and what does it mean in terms of fueling your generator?

In the USA, ethanol, or grain alcohol, is primarily made from corn. You’ve probably heard of “moonshine” or “corn squeezins.” Yeah, in its base form the ethanol in your tank is essentially the same 200 proof stuff famous for making people ripping drunk. Starting around the year 2000 manufacturers started adding ethanol to gasoline. It’s a renewable resource and the United States has a breathtaking capacity to produce enough corn to make billions of gallons of ethanol. Ethanol is not exactly a “green” energy source. It is carbon-based and produces pollutants both in the production phase and as a byproduct of combustion.

American gasoline is classified as follows: E0 (zero), which is 100% gas with no ethanol added. Then we have E10 (up to 10% ethanol), which is the most common. In third place is E85, commonly known as “flex fuel” and contains 85% ethanol. Lastly there is E15. It is, you guessed it, 15% ethanol. The EPA approved E15 for sale in the USA but as of right now it is not on the market.

Chemistry class.

Ethanol attracts and holds water. This is not such a big deal if the water concentration is small and the fuel is cycled often. But when gas sits around, the alcohol attracts more and more water to the point that it actually separates out of the fuel. Chemists call this process phase separation and it is real bad news if it happens.

Normally the water will bond with the ethanol, be combusted in the engine and blow out the exhaust as steam with no issues. But if phase separation occurs, the water is no longer bonded to the ethanol. It’s just pure water not bonded to anything. Of course, if pure water gets sucked into the engine, you are done.

Another attribute commonly cited as being a problem for ethanol blended fuels is that the ethanol accelerates the decay of plastic and rubber parts used in small engines. This is not nearly as true as it used to be. Any engine made in the last 15-18 years or so was designed with ethanol in mind and should be ok. The problem is that there are a lot of generators and power tools older than that still in circulation.

The last bit of bad news: Since alcohol does not have as much energy as E0 gasoline, you’ll need more E10 to produce the same power you would get out of an equal amount of E0.

So what should you use as generator fuel?

If you have a generator less than 15 years old, then using E10 is probably ok as long as the fuel is not allowed to go stale. In general, the problem with E10 fuel is not so much the ethanol itself, but the fact that it attracts and holds water. The only way around this is not to let it sit around long enough to do any damage. Do not use additive products such as STP or HEET as they are nearly 100% alcohol. Introducing more alcohol into your engine will just attract more water.

There is almost universal agreement that any gasoline over E10 should never, ever be used in a small engine unless that engine is explicitly designed for the higher ethanol concentration; some manufacturers are going well out of their way to warn consumers. E15 is not widely available yet, but once it is, pay attention to what you are buying. The same goes for E85. If you live in the state of Maine you have it easy…gasoline with an ethanol content greater than E10 is not permitted for sale there.

If you insist on using only “real gas” for your generator, E0 is available but you will have to hunt around for it and expect to pay a premium. One filling station near my house sells “high performance racing fuel” for about $4.00/gallon. It’s really just ordinary E0. Some hardware stores sell gallon cans of E0. Most marinas and all airports have it too. You might also want to ask your local fire department or public safety agency if they know where to get E0 because many of them will not use E-anything in their power tools.

Passing the gas.

The only way to avoid the E-stuff and fuel stabilizers and rotating and all the associated drama is to not use gas at all. Generator fuel does not have to be gasoline.

Gas generators can be adapted to run on propane or natural gas. Propane & NG will not go stale, do not need to be rotated, and will not leave gunk in the guts of your engine. If you do not already have a generator, when you go shopping consider spending and extra $100-$200 for a dual fuel or tri-fuel version. Dual/tri-fuel engines can easily switch between fuels. If you already have a gas generator, fairly easy DIY tri-fuel retrofit kits are available on line. Other than the added up front cost, the only downside is that you will need to do some plumbing work in your house to use the natural gas side.

Diesel generators also offer an escape from gasoline hassles. Diesel fuel does not have any ethanol (yet) and is much more shelf-stable. The bad news is that diesels in the 2000-10,000 watt size that off grid amateurs like are hard to find and very costly.

What we learned today.

  • Gasoline has a finite shelf life and should be treated with a fuel stabilizer and rotated regularly.
  • Gasoline should be stored in tightly sealed containers with as little excess air volume as possible.
  • Ethanol blended gas comes in 10% (E10), 85% (E85), and the very rare 15% (E15) concentrations.
  • Water is chemically attracted to ethanol.
  • Phase separation is what happens when there is too much water for the ethanol to hold.
  • Ethanol blended gas greater than E10 should never be used in small engines unless explicitly approved by the manufacturer.
  • Propane and natural gas can be used as generator fuel via a tri-fuel adapter. Some generators come with the adapter factory installed.

14 thoughts on “Gas Up The Generator? It’s Not That Simple.

  1. Richard Bender

    Very comprehensive and accurate discussion of fuel – good job!
    I live offgrid 6 months of the year, so generators (and other small engines) are an important part of my life, especially when the sun doesn’t shine.
    For years I’ve been buying “no-ethanol” gasoline for my small engines AND adding Sea Foam (1 ounce/gal), even if I’m going to use the fuel immediately.

    In addition, I winterize by closing my fuel-lines (where possible) draining my carbs (4-stroke) and fuel tanks (2-stroke) and running the engine ’til it quits.
    (I also change the oil (4-stroke) and charge the batteries of the few engines that have electrical starters.)

    No “control group”, so not scientific – but my success has been great!
    In the spring my engines typically start on the first or second pull, and it’s been years since I had to tear-down and clean a carb. All in all, I think your recommendations are spot-on.

    1. Chris Warren Post author

      Your suggestion for adding a preservative even if you plan on using the gas quickly is excellent. Plans change or you might have to mix different cans of old and new gas together. Adding gas treatment to everything just makes things more consistent.

      Draining fuel systems and carburetors is a good idea too. After all, gunk can’t form if there is no gas in the system in the first place.

      Thanks for your insight and comments. I hope you’ll stop by Off Grid Ham again.

  2. DragoSapien

    This is one of those topics that people rarely think about. I’ve learned to use propane myself. My larger generator that runs my RV has already been converted to propane. Because I have an RV I have six 12 gallon bottles stored away. I also have a second little bitty generator that is still gasoline but I will be converting it to propane and that’s what I use for my ham radios. I learned a few years ago after a major ice storm here in West Texas gasoline and diesel are not easy to get your hands on. The ice was so bad that powerlines, radio towers, and all roads were closed. Our city found out that when this happens in over abundance of refugees get stuck here. Because there was no cell phones Internet or electricity people couldn’t get things to eat or put fuel in their vehicles. When the power came on after three days there was no fuel left in this town. People were still here for days waiting for more fuel to come in if the roads would get any better. Even emergency services couldn’t get around to do their job because of the lack of fuel. BUT!!!! Propane was plentiful. My mobile home is all electric so I had no power for the furnace. But I did have a portable in-house propane heater and that sure saved our butts. Being that the RV runs on propane that was another source of heat, cooking, and bathing. Propane barbecue grills can be of some help for cooking or heating water up to. If I could make my truck a dual fuel, I would do it.

    1. Chris Warren Post author

      You make a very good point: In an emergency situation, the most popular fuel will always disappear first. I have a buddy who just managed to make it through Hurricane Michael relatively undamaged. He said the gas stations were running out of gas but you could get all the propane you wanted. Having a dual or tri fuel generator gives you more options, and in an emergency, options are like gold.

  3. Steve

    Hello Chris,
    I first met you some years back at the Hamfests in Wheaton and want to say thanks again for all of the excellent info you share.

    Some years back, at the annual Midwest Renewable Energy Fair outside Stevens Point, Wisconsin, David Blume would present workshops about how to use alcohol as a fuel for small engines . He would have a working small engine running on alcohol on display.
    If you have never read his thick “bible” on how to do this, here is the amazon link:

    Just wanted to mention this in case you were not aware.

    The annual 3-day weekend MREA Fairs usually offer an “Alternative-Powered” vehicles/engines area. If you’ve never attended one (unfortunately they are usually the same weekend as the Hamfest in Wheaton), you might want to check out their website:

    Thanks again for all of your efforts!

    1. Chris Warren Post author

      Hi Steve, I do remember having a lengthy conversation with you at the Wheaton, IL hamfest. I’m glad you are enjoying Off Grid Ham and have benefitted from what I am doing here.

      I am not familiar with Blum’s book but I can make a few observations based on the lengthy description and reviews on Amazon.

      First and most glaring is that ethanol is very expensive to purchase because it is not sold on the retail market in bulk quantities the same way gasoline is. You can buy denatured alcohol at any hardware store but it will cost you about ten bucks a gallon. There are also serious legal issues regarding home production of ethanol.

      Blum’s book was published in 2007 and does not appear to have been revised or updated since then. He is selling information that is over a decade old. That’s not necessarily a deal breaker, but I would expect a “world renowned expert” to be more current.

      Lastly, if it were really that easy everyone would be doing it. I’m not trying to whizz on your suggestion nor trash a book I admit I haven’t read, but I am skeptical of the practicality (and legality) of what Blum is pushing. Off Grid Ham was founded on a “tell it like it is even if it aint pretty” philosophy.

      Thanks again for your input and for being a loyal reader.

  4. randall krippner

    Nice article, Chris! You covered all of the important points. I wasn’t aware that some generators could be retrofitted to run on propane. I’m going to have to look into that and see if my Generac can be converted to bi-fuel or even tri-fuel.

    A few words about straight alcohol as a fuel — Yes, you can convert small engines over to run straight ethanol but that doesn’t mean it works very well and it certainly doesn’t mean it’s a good idea. Ethanol has some serious issues when used as a fuel. It has 30% less energy per unit of volume than gas so you have to burn considerably more of it to get the same amount of energy as gasoline. Ethanol is water soluble, so it can pick up contaminants that gasoline may not, causing fouling of your fuel system, injectors, carburetors, etc. It can also cause rusting of the inside of improperly coated fuel tanks because of water contamination. You need to change the fuel/air mixture. I.e. you have to adjust the fuel system to run “richer” so the fuel/air vapor being fed into the cylinders contains 25 – 35% more fuel than with gasoline. Modern cars/trucks have sensors that do this automatically but with small engines you need to adjust the carburetor yourself. As you pointed out, it’s hygroscopic, meaning it absorbs water from the air, so it needs to be kept in airtight containers.

    And there is the cost. Ethanol is, frankly, cheap. That’s one of the reasons why ethanol blended gasoline is less expensive than straight gasoline. *But* if you want to buy straight ethanol, the price skyrockets, as you found out. Some of that dates back to government regulations instituted during the Prohibition, some of it is simply price gouging, that kind of thing. Yes, you can distill your own alcohol. The process isn’t difficult. But it is illegal to do so in almost every state in the country without going through an extensive, expensive licensing process, with regular audits, inspections, etc. Again, that goes back to laws that date back to Prohibition. Basically they’re afraid you’re going to let people drink the stuff. There are loopholes and gray areas where you can get away with distilling very tiny amounts of it. But to produce enough to feed an engine, you need permits, licenses, inspections, audits… Unless you’re going to do it on a commercial basis it isn’t worth it.

    Thanks for the information Chris!

    1. Chris Warren Post author

      I’m glad you got a lot out of this article. Ethanol shows a lot of promise but it is not a miracle solution. Ethanol supporters, cheered on by the agriculture lobby, tend to oversell the benefits and gloss over the disadvantages.

  5. Pingback: Winter is Nearly Here… some tips for your generator – Coast Emergency Communications Association

  6. randall krippner

    I think that one thing the article and subsequent comments indicates is that backup generators can be very useful and important, but in the long run they are just a stop gap measure that will only be useful for a relatively short period of time. A truly significant event is also going to disrupt the supply of fuel of any time being delivered into the area if it goes on for more than a few days. In all likely hood even the natural gas supply, which a lot of hospitals and other emergency services depend on for their backup power, is going to be disrupted as the NG supply is often shut down deliberately to prevent fires and explosions. A house about 6 miles from here just exploded — literally exploded, because of a natural gas leak. There wasn’t even a decent bit of firewood left. Even here about 6 miles away the explosion rattled our whole house.

    The only truly long term energy supply under those conditions is going to be solar panels and perhaps small wind turbines.

    1. Chris Warren Post author

      You got it exactly right. In addition to fuel supply concerns, most generators are not meant for long term use. They are built to meet a price point and manufacturers know most people will not run them often. This is not always a bad thing, depending on your perspective. For example, building contractors actually like cheap generators because they are often stolen or the work crews will beat the hell out of them. Spending a few hundred bucks to replace a generator every few seasons or so is just the cost of doing business. That’s why you never see $3000 Honda generators on construction sites. Off grid hams, survivalists, and the like want a machine that will last and last. Unfortunately, quality is hard to find even if you are willing to spend the money.

      Wind and solar have their longevity issues as well. After all, batteries have a finite lifespan. My battery bank is past the end of its useful life and needs to be replaced. I’m hoping to squeeze it a little longer until I get my tax return as it is a very expensive fix. Obviously, solar/wind are a more long tern solution than mechanical generators, but no one should think anything is forever.

  7. spacecase0

    I got 5 gallons of E zero gas about 12 years ago in a metal gas can, was 87 octane.
    used it for my tiller every year to till up the garden
    ran the tiller dry every year, so no gas to mess things up as it waited to next year.
    at about the 10 year range I started getting knocking from the engine when it was cold.
    turns out that is just what happens with old gas.
    I then got a truck, it was free, and totally run dry of gas. Back taxes from the CA DMV made it worth less than nothing, but I just wanted a farm truck, so I got a nice truck in good condition for the only price of giving it a good home.
    it had been sitting for a year before I got it
    so I put my last 2.5 gallons of now 11 year old gas in it.
    it started and ran.
    but idle was not so good. had to keep the gas peddle on a bit to keep it running.
    after I ran that out, it ran perfect on fresh gas.
    I only share this story because I had so many others tell me that what I did with no additives at all was not possible.

    something else I have found out is that higher octane gas is harder to make, so… they often put in more additives to get it to a higher octane.
    because of that, the lowest octane you can get is often the purest.
    many ears later I got verification of what I had learned.
    one of my friends only wanted the best for his small engines, so he only filed them with 91 octane (the highest you can usually get in CA).
    he had come to the conclusion that a carburetor only lasted one year….
    so he just bought new carburetors for all his small engines every spring.
    I had known already from model airplane people that 91 octane or higher only stores well for very small engines for only a few days, but a car will burn it just fine.

    the lesson I got with that is to not store gas in the tank of the engine (already knew that)
    and if you start with pure gas, you can have something running far into the future as long as it is air tight and you do not pointlessly get higher octane than you need.

    1. Chris Warren Post author

      I’m doubtful your results could be replicated with any consistency, and I don’t suggest leaving untreated gas in a can for extended periods with the expectation that it will be ok. I think you were just lucky. Also, as you observe, some engines are more tolerant of “horse whizz” gas than others.

      The best way to get rid of stale gas is to add a gallon or so to a nearly full car fuel tank. A small amount diluted into a large volume of fresh stuff should not hurt anything.

      1. spacecase0

        I have about 20 gallons of E0 with a 91 octane rating in storage that is about 12 years old.
        it is in plastic gas cans…
        so I get to check if I can replicate my previous test.
        but I am not that dim, I would not trust it to work.
        if it fails in the riding mower I will burn it bit at a time in my truck as you suggest,
        I am sure not going to try it in my car as I can’t replace the car.

        even if the old fuel works, pretty sure that I could not try this test again.
        I have not seen E0 for sale at a gas station in CA for about 11 years now. and that is why I stopped rotating through what I have now.

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