What's the Difference in Generators?
April 4, 2021 8:27 AM   Subscribe

Almost six years ago, I bought a Generac XT8000 portable generator for $1300 from Lowes. It has an 7 gallon gas tank and can run for 11 hours at 50% load. Until last year, the most I ran it was a monthly, 15 minute test and occasional power outages (maybe three times a year) lasting up to or totaling, maybe 12 hours.

Western Oregon's recent ice storms, and its wildfires last year worked it hard. It ran two days straight during the latter and 8 days straight for the former. When the generator repair tech looked at it, he said compression was down from a factory spec of 150 psi to about 60. He said no consumer generator is designed to run continuously for that long. He said mine would eventually not function with loads, backfire b/c of worn down pistons and that I should think about replacing it.

Costco sells the Generac XT8500EFI for $999. It has an 8 gallon gas tank, but its specs say it can run for only 9 hours at 50% load. The tech said all portables run at 3600 rpm and only the load changes how fast it burns gas after that. I never ran my generator over 33% of full load. I'm thinking about getting the Costco generator to replace my current one, but I don't understand why, if both generators run at the same speed with the same load, why would the new one cost less, have more potential power but be less efficient (seems to burn more gas) than the old one?

I thought that maybe, I should buy a home standby generator (I don't care about all of the bells and whistles, like calling me, autostart, etc. He said that's more stuff that can go wrong and I agree). But he also said consumer level standby and portable generators, most of which have single stroke engines, are essentially the same and neither are designed to run for a week at a time. Is he right?
posted by CollectiveMind to Technology (15 answers total) 6 users marked this as a favorite
Single-piston engine, perhaps; not single-stroke. Most are 4-stroke. And most small engines this size do not have oil filters, and require the oil be changed every 20-25 hours. For things like lawn mowers, this is perhaps once per season for the homeowner. For a portable generator running continuously or nearly so, this could be daily.

Regular oil changes are particularly important on small engines like this, as without an oil filter, contaminants and metal shavings just remain suspended in the oil. Many brands sell a magnetic dipstick (or you can just use a small magnetic pickup tool) that aids in collecting metal shavings from the bottom of the oil pan. Getting these metal bits out when you change the oil will greatly extend the life of the engine.

Motor oil breaks down with use, and without regular changes, this plus the suspended contaminants will result in engine wear. If your generator was run for a week without an oil change, it is entirely possible that you've lost a good deal of compression. You may be able to remedy this with an engine rebuild; a light cylinder honing and a piston & rings kit can restore a lot of life to a worn engine.

Generators that are powered by small engines like this work best and last longest when they are run for a few hours at a time, and with proper maintenance and oil changes as recommended. Most of these consumer generators are just air-cooled; there's no liquid cooling the engine (like the radiator in your car) and almost certainly no oil cooler. Heat is the enemy of machinery, and without an active cooling system, long and sustained running of these engines will definitely shorten their life. There are larger generators, with engines that have oil filters and coolers, and engine coolers, that are purpose-built to run continuously. But not these portable consumer generators.

How long a generator will run on a certain amount of fuel is a factor of its engine size and configuration, the generator itself that the engine is powering, and the load. Just because one runs for less time on the same amount of fuel doesn't necessarily mean it's worse. Could be a larger engine, could be a more capable generator, who knows.

Whether it's worth upgrading to a model that is much more expensive, but built to run continuously, is up to you. You've got not only the upfront equipment cost, but also regular maintenance just like any other machine. It still needs regular fluid changes, regardless of hours run, just like a car. There's always a solution; if it is absolutely critical that you have 100% uptime for power, there are companies that can certainly provide that capability, but it's sure gonna cost ya.
posted by xedrik at 8:52 AM on April 4, 2021 [1 favorite]

The spec sheet for the XT8000E says it has a 420cc splash-lubricated engine, a rated output of 8kW, and will run for 11 hours at 4kW (50%) output on 7.5 gallons of fuel.

The spec sheet for the XT8500EFI says it has a pressure-lubricated engine with electronic fuel injection, a rated output of 8.5kW, and will run for 9 hours at 4.25kW (50%) output on 9 gallons of fuel. It doesn't say what the engine displacement is but it's probably the same 459cc model used in the 8kW XT8000EFI, which is rated to run 9.5 hours at 4kW (50%) output on the same 9 gallons of fuel. It's a bigger motor than the one in the XT8000E and that, plus maybe a bit of power loss for the oil pump, is probably enough to account for its lower fuel efficiency.

The pressure lubricated engines do have oil filters, and the owner's manual for the XT8500EFI says that the oil should be changed every 100 hours of run time, or every season, whichever comes first. I would expect a splash lubricated engine to want more frequent changes than that for the reasons xedrik explains.

If you avoid running the new machine eight days straight on five year old oil, I would expect it to retain good compression for rather longer than the old one did. Probably wouldn't hurt if one of the loads you connected to it was a box fan blowing over it, too.
posted by flabdablet at 9:16 AM on April 4, 2021

Also, if two engines with the same power output at the same RPM (4kW at 3600, in this instance) have markedly different fuel efficiencies, the one with the lower fuel efficiency probably has a lower compression ratio and will therefore be a little more wear resistant.
posted by flabdablet at 9:25 AM on April 4, 2021

I can not speak to the differences between the two generators, but the new one likely has a more powerful engine that is just not as efficient. The MSRPs on Generac's website are only about $100 different, Costco probably just bought a lot of them and are giving a good price. Two things I will note are to make sure you are using ethanol free gasoline in your generator and add a stabilizer/cleaner to the gas like Marvel Mystery Oil or Sea Foam.

Another suggestion is that if you never had more than a 33% load on a 8000W generator, why do you have an 8000W Generator? If you want to replace yours, you could go with two in the 3500W range and save a bunch of money and fuel, plus have more flexibility and allow for oil changes for extended use. For example, Costco has a Firman 3650W right now for $500 that would seem to fill your needs, has a 5Gal tank that runs 14 hours at half load. You could get 2 and take turns running them in an emergency. Or for the 2nd one you may get away with a smaller inverter generator that is a lot quieter and uses a lot less gas during the night.

I have a 7500W for Hurricane cleanup and a 1000W inverter generator that will run a few lights and fans and one refrigerator at night.
posted by Short End Of A Wishbone at 9:25 AM on April 4, 2021 [1 favorite]

I was reading specs for both on GeneratorBible.com and they are practically the same



So the only difference is as flabdablet explained: splash vs pressure oil system. And apparently running it continuously had caused some premature wear on your engine.

Here's Briggs and Stratton explaining the differences between the oil systems.

posted by kschang at 9:35 AM on April 4, 2021

The XT8000EFI has (I think) the same engine as the XT8500EFI, the one that's both larger and less fuel-efficient than the (splash lubricated) one in the XT8000E.
posted by flabdablet at 9:56 AM on April 4, 2021

No. What are you going to do for the zombie apocalypse.
A diesel standby generator is your best choice. Make sure you intergrate it manually or automatically to your main fuse box. It should be large enough to cover your largest wattage draw. For our use it would be enough to run a well and
Electric welding. I have lived off grid for many years: if you go cheaper gas models
Don't buy Briggs and Stratton. Honda or Yamaha bought not from a big box store.
I have used generaters every day for years.
posted by JohnR at 10:07 AM on April 4, 2021 [2 favorites]

I am not up for the ongoing maintenance that either type of generator demands, so I got an inverter that will allow my car engine to function as a generator. This may be an option for you depending on your vehicle and power needs. (I don't need 24/7 power.)
posted by metasarah at 3:43 PM on April 4, 2021

generatorstore.com.au has the following to say:
If you need to run your generator continuously for more than a few days, a portable generator simply won’t cut it – you need a standby generator.

Standby generators have much larger and more efficient motors that are specifically designed to handle long-term use. They’re also designed to operate on diesel, with the generator model usually having huge long range tanks.

If you are running a portable generator on a relatively unlimited supply of propane, you’ll be limited to the life of your engine oil. That gives you up to 200 hours, or about eight days, worth of continuous power.
I am guessing you didn't have time to do an oil change between then, and that caused the premature wear.

So the real question here is... Should you just keep using the portable generator for a few hours of power... Or upgrade to a genuine standby generator that provides days of power?
posted by kschang at 7:41 PM on April 4, 2021 [1 favorite]

Response by poster: fabdablet, I built a special enclosure for the generator which had a big air inlet beside the generator at ground level and a big industrial exhaust fan in the top of the enclosure that was plugged into the generator, such that when the generator ran, the exhaust fan ran.
posted by CollectiveMind at 7:42 PM on April 4, 2021

Response by poster: ShortEndofaWishbone, Originally, I got the 8KW b/c I thought it could run a furnace or a stove, until I learned the difference of single vs three-phase power. Afterwards, I decided it was a good reserve.
posted by CollectiveMind at 7:45 PM on April 4, 2021

Response by poster: xedrik, you're right. I mean four stroke, single piston engines. Not single stroke.
posted by CollectiveMind at 9:32 PM on April 4, 2021

Before you buy a hardwired standby generator consult with your electrician about their recommendations. It needs to be specced right for your home and a direct connection to 240v line.

We have a 1000 watt Champion dual fuel that works great for us, with regular power failures a thing. I’ve run it for as long as 3 days, but with an oil change every 50 hours as per manual. Changing the oil during a freezing storm is No Fun.
posted by spitbull at 2:50 AM on April 5, 2021

You shouldn't need 3-phase for running a residential furnace or stove, unless we're talking something huge and industrial. Most residences don't even have 3-phase service. 240V, sure, which many generators will supply, but not 3-phase. And if you really do need to run a big motor off 3-phase, you might be able to instead use a VFD on standard supply.
posted by xedrik at 7:24 AM on April 5, 2021

FWIW, standby generators like the 22 kW Generac Guardian are said to have a typical lifespan of 3000 hours running. The unit itself is $4500, so that's about $1.50 an hour! It's just an engine, and presumably with maintenance and parts replacement could be made to run longer. But they are not designed for heavy continuous use.
posted by Nelson at 10:42 AM on April 5, 2021

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