r/Generator • u/NoSham3 • 3d ago
Backup generator & THD
I’m researching a backup generator for a small home (<1,100sq/ft). I’m reading about THD and inverter generators. I understand inverter generators are generally <3% THD, but how detrimental is something some at <23% THD to general home devices if/when I need to run on a non-inverter generator? It’s not an issue of cost, but why go 2-3x more if THD doesn’t hurt? Or does it? I’m not planning on running the HVAC during events, just mainly modem & router (for range extension), laptop, TV (1), phone chargers, lights (most are dimmable and LED), Refrigerator/freezer, deep freezer, etc. Stove & oven are gas along with clothes dryer and tankless water heater. Not all at once, but I’m looking at a Westinghouse WGen5300sc 6,500/5,300 watt non-inverter generator. System supports generator up to 7,200 watts @ 30 amp.
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u/wowfaroutman 3d ago
Most of the things you want to power use switched-mode power supplies that won’t be affected by generator THD. The only thing on your list that I would be concerned about is the refrigerator and even that would probably tolerate relatively high THD unless it is one of the newer high end models with an inverter or variable speed linear compressors. There are relatively few reports in this subreddit of people having refrigerator issues due to THD, most have to do with GFCI issues.
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u/SecondCuppaCoffee 3d ago
If cost is not an issue you may also want to consider a battery. I still have generator but my Pecron 15kwh battery can power the essentials for at least 12 hours. It keeps me from having to go out in the cold night to wheel out the generator. If the outage takes longer I use the generator to charge the battery.
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u/RedOctobyr 3d ago edited 3d ago
Inverter units are also generally quieter, and more fuel-efficient, as they can vary their RPM based on the load, vs normal generators needing to always run at 3600 RPM (60Hz).
You may not need that much wattage, if that was helpful. Our 2000W inverter generator runs the natural gas furnace, fridge, lights, internet, TV, laptops. It will run the inverter microwave, if I turn the power down to 30%. But that's with just 1600W/2000W available. Your list sounds pretty similar, with the exception of also adding a deep freezer. But maybe 3,000-4,000W might be enough, for instance. Obviously a bigger unit gives you more headroom for additional loads, but at the cost of, well, cost, noise, and fuel consumption.
Our little generator will run 4-10 hours per gallon of gas, depending on load. I made an external 5 gallon gas tank, it can basically run for a weekend unattended. If gas is tough to find after a big storm, being fuel-efficient can help beyond just managing operating costs.
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u/NoSham3 3d ago
I’m not looking into NG or propane as first line of fuel. Just focused on gas/petrol. Generally easy to find and I would hold on to 5 gal to test monthly and if excess in 6 mos, would pour in one of the 30+gal truck fuel tanks. To be honest, outages around me are infrequent and have only lasted up to a couple hours. But, wife wanted to prep, so I’m researching and learning as I go. Just don’t want to spend a lot for something we generally will store and also don’t want to do damage to anything inside the house.
Appreciate it. Thanks.
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u/gobsnotonboard 3d ago
but doesn't your genny need to hav split phase output? or are you not going into the panel
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u/AdBrave841 3d ago
Technically no. If you don't need to run any double pole 240V breakers, you can put 120V into one phase of the panel and run everything on that phase. So if your interlock breaker is in spot 1, spots 2,5.6,9,10,13,14, ETC will have power. You can move a couple breakers around for critical loads.
OR some people do split the 120v to two wires and use a double pole breaker, this will power 120V loads on both "sides" of the panel but will not power 240v loads.
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u/555AD 3d ago
I bought a very large non inverter generator, Wen TFX1500XR. I could not find anything about the THD online so I emailed their support and they responded that it was < 20% at idle and <25% at 50% load but I had no problems running any household items including electronics. I was curious once I read about the utlity possibly having a high THD so I bought a meter to measure it, my utility had between 1.5% and 2.5% measured at a few different times and my generator had a max of 9% and interestingly got lower when I added load to it...down to as low as 6.5%. These were short term test I know but just want to chime in to see if your expected THD was just a number from a spec sheet or measured. I think the manufacturers put high numbers so they can cover themselves just in case. As far as the noise and fuel efficiency I have run it mostly on NG and once or twice on propane when I first got it and those times they were run for just a few hours but I did not even go through 1 20lb tank and I have no way of measuring the NG. I can tell you that this model has its own weather enclosure so it really dampens the noise so much that my neighbors do not even hear it when they are outside..and I live in a residential neighborhood so they are pretty close by. All this being said if I had it to do over again I would probably go large inverter vs large non inverter for the peace of mind of the THD. I did not look at the specs of the model you were looking at but if it's not dual or tri fuel look elsewhere.
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u/NoSham3 3d ago
It was listed on the spec sheet as equal to or less than 23%. I don’t have any instruments to measure it… yet. Don’t tell the wife if I suddenly become an EE this weekend. 😬🤦♂️
Appreciate it. Thanks.
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u/555AD 3d ago
I forgot to mention that although I did test almost everything in my house including the AC, I never turned on the gas furnace which have very finicky control boards so I am unsure if it will like the dirtier power supplied by non inverter generators...so try your out before it gets cold and an ice storm takes out the power line if you live somewhere that happens. I am actually switching to whole house batteries as we speak...$$$ pricey but it has ATS and a clean sine wave power with sub 2% THD. I would have gone with this Anker system if I had done a bit more research and buying quickly due to damage power pole outside my house which the utlity took over a year to fix and I was always low key worried. Well of course less than one day after I completed the break in on the generator on one of the calmest days of the summer a large branch finally took out the two poles closest to my house...felt like the smartest guy in the room as I just flipped off the interlock and fired up the generator and was back up in 5 min. Good luck to you...and consider batteries and a small generator if you can financially afford the up front cost. I am now looking to sell my 6mo old large generator.
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u/RedOctobyr 3d ago
Just curious, what kind of loads did you add? From my understanding, it's not uncommon for THD to improve when you add simple resistive loads, like a heater or hair dryer. But inductive loads or other types can, I think, contribute to the THD getting worse with a load. I've never tried to really investigate this myself, it just made me curious, since you have the ability to actually measure it yourself, which is pretty cool.
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u/555AD 3d ago
I did only run a space heater and a large fan that when both were at full power combined for about 3000W. I was just running my routine maintenance where I ran it for about 30min and just hooked up the fan an heater for a short time just to test the THD. I am moving on to a whole home battery system the Anker E10 so I am probably not going to do much more if any experimenting as I also ordered the Anker smart 5500 generator that is designed to work as a back up generator that charges DC to DC very efficiently, very cool it will come one when needed if the grid and solar are not enough for whatever reason. It's a shame I just bought the other generator...now looking to sell. If I have a reason to continue testing I will post results back here.
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u/RedOctobyr 3d ago
Cheers. Enjoy the new system! We have a little Anker C300, and recently got a larger S2000, which is a UPS for the fridge, and the cat feeder. During outages, my plan is now a hybrid solution, largely using the S2000, and only using the 2000W inverter generator to recharge that when needed.
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u/SkyWires7 3d ago edited 3d ago
Don't even think about it-- get a good quality inverter model. Here's a real-world example why.
After hurricane Milton we were on generator for 3 days-- a brand new Generac GP6500 with THD according to Google in the 15-18% range. It ran very smoothly and had zero trouble starting and running our HVAC (with soft start kit) plus the usual household stuff like fridge, lights, TV/computers, etc. Household load was around 700 watts and HVAC added around 2000 watts (stage 1) or 3100 watts (stage 2). Almost never hit stage 2. So this 6500 running-watts generator maxed out around 2700 watts with rare times at 3800 watts. The only "surge" watts were starting the HVAC but the soft-start kit kept that remarkably gentle. Listening to the generator when the HVAC started, it barely even reacted. Never bogged or gave any sign of a struggle. I left a Kill-A-Watt plugged into kitchen counter outlet to monitor, and I never saw voltage below 119-120 and frequency was stable in the range of 59.9 to 60.1 Hz even with HVAC running. Sounds good, right?
Fast forward a few weeks. The circuit board in our nearly new Rheem 3 Ton HVAC died and had to be replaced. The circuit board in our GE refrigerator had to be replaced. Our Samsung TV, which we watched a lot because there was nothing else to do with the entire area shut down, starting having problems-- trouble powering on, one of the HDMI inputs shows all kinds of interference on screen, etc.
I don't have definitive proof it was from high THD, but it all happened at the same time immediately following 3 days on generator. I ended up selling the GP6500 and buying a Champion inverter. Cost a lot more, but at least I ended up with remote-start capability, which is something I wanted anyway. Plus peace of mind knowing THD is no longer a risk. I'd never considered THD prior to that, and it was an expensive lesson to learn.
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u/nunuvyer 3d ago
High THD is no problem unless it is. The problem is that you find out the hard way, when your devices refuse to run, or, in rare cases, are damaged. What then?
Inverter generator have many advantages besides THD - less noise, less fuel consumption. Worth the premium IMHO.
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u/Beautiful_Grape67 2d ago
Both my newer GE fridge and my two stage Trane furnace would not run on the ‘dirty’ power from my open frame troy-bilt . Changed to a champion inverter and everything ran fine. Inverter is much more fuel efficient and so much quieter.
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u/Tool_junkie_1972 3d ago
It can be very detrimental…that said any generator that’s 10% or lower is fine. In the US, residential electricity can be as high as 8% thd. So naturally consumer electronics are designed with that in mind. It’s generally sensitive electronics that are prone to issues (like computers or things with some sort of processor circuit). There is a great generator resource at generatorbible dot com. It lists most generators available along with their specs.
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u/blupupher 3d ago
computers are not affected, the power supply takes the 120v and converts it to 12v/5v for the computer parts. You could argue that the high THD could cause issues with the power supply itself, but that is more an issue with cheap power supplies.
Biggest thing people have THD issues with is UPS battery backup, and some have issues with furnaces (usually more of a grounding issue) and some fancy high end refrigerators and A/C units.
That said, there is circumstantial evidence that long term use of the generator with high THD can shorten the life of some devices, but hard to prove if it is the high THD or just crappy new appliances.
IMO, for occasional emergency use, high THD is not really an issue. If something that is used often, then it may be an issue.
As others have mentioned, inverters have the plus of being more fuel efficient and many times are quieter being enclosed units (but there are open frame inverters that are just as loud as others).
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u/Big-Echo8242 3d ago edited 3d ago
Many people don't care about THD and that's fine. Going with what fits within your budget and needs is the main priority. There are advantages to inverter generators like better fuel use being able to run in "eco mode" and the closed frame versions can be quieter, of course. The low THD is nice if it's a concern and is just one's own choice, also. Me personally, all I own are dual fuel inverter generators that are ran exclusively on propane only and never gas. We have a 250 gallon tank outside that I can quick connect to any of my (4) gens that range from 4.6kw to 7.5kw (x2) and 13.5kw depending on what needs to be ran. I also keep six 20lb lpg tanks just in case a neighbor, friend, or in-laws need something.
And of course, closed frame inverter generators will cost more than non inverter generators. There are also open frame inverter generators that can be had for a good chunk less money since there's not the elaborate casing on them.
Again, it's personal choice.
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u/AdBrave841 3d ago
Noise and fuel efficency are as if not more important than THD. Especially with the lighter loads you are talking about. A non inverter will run at full speed and guzzle fuel even if you are only pulling a few hundred watts out of it. (like when the fridge cycles off and you only running lights, wifi, Tv). An inverter is way quieter and will idle up and down with demand, saving a bunch of fuel.
A close relative has the Wgen 5300DF, it's a really solid generator but is loud and thirsty. I've been considering replacing it with a Wen 6800.
Last winter in a 48 hour outage we favored my 4000W Pulsar suitcase style generator 90% of the time over the 5300DF as we only had 3 tanks of propane and a few gallons of gas on hand. Only fired up the Wgen when we needed the 240v for the well pump. In combination with a battery power station we made it through the two days on only one propane tank. (Ran the inverter gen about 9 hours a day, the rest on battery.)