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Help sizing a generator for home backup?

 
Adam Dettman
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Hi, folks.  Does anyone have any insights regarding sizing a portable generator for home backup?  My electrician is prepared to set me up with a (legitimate, up-to-code) method for backfeeding my panel in the case of a power outage.  I'm barely a novice in terms of electrical work/home wiring and so please don't ask me to provide details regarding his methods or materials.. just know that he is a licensed electrician!

That being said, any recommendations regarding what'd be an adequately sized portable generator to provide backup power for my home?  In terms of usage/load, my goal would be to maintain power to the pumps/computer associated with my propane boiler (home heat + hot water), well pump, freezers (one chest, one upright), refrigerator, air exchange system, and then I guess some lights and probably a laptop/phone charger.  I don't own air conditioners nor would I plan on running power tools during a power outage.  Additionally, I think I could handle the responsibility/"burden" of unplugging my freezers once they got to temp (just meaning that some stuff wouldn't have to stay plugged in/operational constantly).  My primary concern would be to keep my boiler functioning (as this will provide my home heat in addition to my woodstove) and my well pump operational.. as well as my fridge and some lights I suppose.

In light of all this, would a 4,000W generator suffice?  Or is that cutting it close?  Is something like 6,500W more appropriate?

I should also mention (perhaps very important) that my power doesn't go out very regularly.  Furthermore, if and when it does go out it rarely lasts more than a few hours.  And again, this is perhaps twice a year.  I just happen to live in a very cold part of the country (Northern Minnesota) and a modestly priced generator seems like reasonable "insurance" against frozen pipes!

I'm really happy I joined this site not too long ago.  As such, I look forward to hearing what some of you have to say.. Thanks!
 
Ben Zumeta
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Good question. I don’t have any answers for you but have a question on the other side of the coin. I have as my solar backup a 25kw diesel generator that came with my off grid place, and have been told (and intuited) that this is a waste of fuel warranting a smaller generator for basic use on cloudy days. I want to minimize my use of diesel, for many obvious reasons. I get a good amount of sun at this new place (probably 250 full solar battery recharge days/yr). Any thoughts on how to bridge the gap between a beast intended for growing and something I can use to keep small electrical appliances running during the dark winter months?
 
Kate Downham
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I wonder if getting more solar panels and batteries would do the job?

Last year we had the option of either replacing our generator, or spending a little bit more to get some good solar panels and a charge controller so that we didn't need a generator.

With generators there is more than the fuel cost to consider - they break down, and where I live there's not many people around that repair them, and those that do charge a lot of money. For occasional backup use it still needs to be maintained, and can still fail when it's most needed, so I'd rather just use less electricity in winter rather than have a stinky noisy generator.

I'm not sure of the wattage of the things you want to use. It might be worth getting a kill-a-watt type tester thing and seeing what the different appliances use. Fridges and freezers have a big startup surge, so to reduce the amount of watts needed in an inverter or generator it can be a good idea to switch them on one by one, so that they don't surge all at once.

With our solar, we switch the freezer off when the sun goes down, and switch it back on again when there's enough watts coming from the panels, and it still works well, even in the heat of summer.
 
Eric Hanson
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Adam,

Good job on getting s legit backfeed panel and not trying to go cheap.  I too have a backfeed panel with dedicated “break before make” switching to ensure that I never accidentally try to send power out of my house and down the main line.  That kills electrical workers.

My house is fairly conventional so I have fairly typical power needs.  I have turned on my generator and thrown the switches in the adjacent sub-panel and I was able to power s good part of my house with no problems.  Something worth considering though is the microwave.  My fairly typical microwave needs about a full kilowatt for normal operation.  Turning on the microwave really makes my generator surge.

For reference sake and to more directly answer your question, I am running a Generac GP5500, a 5.5 ke generator with a 6kw surge capacity.  This is more than adequate to run my all electric house—minus things like the heat pumps, hot water heater, dryer, etc.—until I turn the on the microwave and all of the sudden the generator runs to near full capacity for 30 seconds or however long the timer is set.  Other than the microwave, the circuits that are dedicated to run in my house on generator barely pull 2-3 Kw until I turn the microwave back on again.

Long story short, if you plan on being able to use your microwave, I would suggest about a 5-5.5 Kw generator.  Fortunately these are common and reasonably priced.  Mine has a 6.5 gallon tank and will run for over 10 hours at a 50% load.  Smaller than this will still be adequate for much of your house, just don’t use the microwave.  Anything larger is a waste IMHO unless you plan on running really high amp devices like heat pumps, ac units, etc.

I hope this helps,

Eric
 
S Bengi
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Can you let us know how many KWH you use per month? I am going to go with 900KWH/month and 30KWH/day. 20% of that is 6KWH (50A x 120V).  50A is usually the cutoff point for quite a few things too.
A 6KW continuous generator is going to cost you $5,000, you will then need fuel too, about 24gallon a day of fuel.

Lights, electronics, furnace blower fan, fridge/freezer and microwave/toaster only use about 3KWH of energy per day. and right about that amount concurrently.
So if you cut back you can just plug a 4000W inverter with a surge of 8000W into your system. You can charge it up with the grid or the sun.

$150 Solar Panel 2000W and 8KWH/day https://www.wholesalesolar.com/1977433/astronergy-solar/solar-panels/astronergy-chsm6612p-hv-345-silver-poly-solar-panel
$500 Charge Controller 80A https://www.wholesalesolar.com/3510605/outback-power/charge-controllers/outback-power-flexmax-fm80-charge-controller
$2000 Battery 4000W https://www.batteryspace.com/lifepo4-rechargeable-battery-12-8v-100ah-1269-7wh-100a-rate-with-bluetooth-option---un38-3-passed.aspx
$1000 Inverter 4000W  https://www.wholesalesolar.com/2923535/cotek/inverters/cotek-sp4000-124-inverter
Total Cost = $5000

Yes this system would limit you to just 4kwh-8kwh per day. But this only once every blue moon in an emergency situation.

I am not sure what setup your contractor is using but take a look at this.
https://www.homedepot.com/p/Reliance-Controls-200-Amp-Generator-ready-Loadcenter-with-Meters-TTV2006C/202216486

I also like this one, your electric company will even install it for free
https://www.homedepot.com/p/GenerLink-40-Amp-Meter-Mounted-Transfer-Switch-MA24-N/301962163?MERCH=REC-_-visuallysimilar_1_0-_-301962200-_-301962163-_-N
 
thomas rubino
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Hi Adam;
I would recommend a smaller unit. As Eric has said' a microwave is one of the heaviest draw items in your home. Avoid using it while on generator if you can.
Other than that what you have will run on a much smaller genset.
I highly recommend getting  an inverter generator.  Honda and Yamaha are the best known makes, they also are very , very expensive at $2-3000
Harbor freight makes a 3500 watt Predator inverter generator.  Ave cost is 600-700$   Super quiet, super efficient. They idle down when the draw is low.
You might think that 3500 is not enough. With a regular genset it would not be.
I used up 2 3500 watt Onan gens in no time They barely made enough power and were  sitting there screaming at 3600 RPM's! They cost close to $2500 each!
The harbor freight unit handles anything I throw at it and it does so Quietly while sipping propane !!!

I have had my H.F. Predator now over two years. I live off grid and the gen gets run often depending on how sunny it has been. It has been flawless!
I converted mine to run on propane using a great kit available from US Carburetion.  No fuel filling in the dark,  no old fuel gumming up the carb. Engines run clean and last longer on propane AND its cheaper than gas!
I highly recommend both the predator and the propane conversion kit.
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R Scott
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You can't use an inverter generator to backfeed an existing panel.  The genny is 120 but the panel is 240.  There are a couple Chinese models that will sync two genny together to make 240, but the name brand sync kits just make a higher amperage 120 output.

Your well pump is probably the hardest starting load.  I have a 4000w genny and it could start a small shallow well pump but barely.  

My plan for the new farm is to get a 10k welder generator because I need a stronger welder anyway.  But for just rare use backup power I would get a 6-10k PROPANE generator, reason being they can sit for months and start right up, no fuel going bad or gummed up carbs to clean at the worst possible time. If you can get propane to where you want it cheap and easy enough.

As for a 25kw genny, you could get a smaller genny or a bigger charger.  How fast can you charge your batteries? It could be cheaper to max your charger and cut the run time you need.

But I think every homestead needs a 2-3k inverter genny sitting there for backup or fieldwork.  Cordless tools are awesome, but there are times when dragging a genny and corded tool out to the field still makes sense.
 
S Bengi
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Alot, maybe most generator in the 6000W and above does provide 240V, they usually have a 4 prong outlet, you might be used to that on RV's and boat slips too.
https://www.electricgeneratorsdirect.com/Honda-EU7000IS-Portable-Generator/p14017.html

They usually come in 30A and 50A


You can even plug it into your electric meter. Without the hassle of having to re-wire your house.
https://www.homedepot.com/p/GenerLink-40-Amp-Meter-Mounted-Transfer-Switch-MA24-N/301962163

 
James Freyr
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Hi Adam!

Regardless of what brand and size you choose, I recommend looking at the Total Harmic Distortion made by the generator. I just wrote about the other day in this thread here: https://permies.com/t/137115/Backup-power#1075349

Here's a cut and paste of my post in the other thread:

Yes I have a recommendation, from something I learned the hard way. Look at the Total Harmoic Distortion (THD) output from the generator. THD kills sensitive electronics, like oven and refrigerator controls, digital thermostats, new digitally controlled washers and dryers, the control panel in modern furnaces, anything digital. THD is often what makes two generators of the same size vary in price by a huge margin. Good generators that are make minimal THD like under 5% are pricey. Generators that make more than 10% or more THD tend to be much more affordable in price on the front end. When I was researching generators, I was so focused on watts, and running watts, and surge watts, and I had no idea THD was a thing or to even look for it. It is labeled for every generator, somewhere either on the generator box, in the manual, on the manufacturers website or on the side of the generator itself.



I hope this helps you make a decision!
 
John F Dean
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The answers here are pretty solid.  You need to determine if you want to back up everything or just essentials.   If you have a modern fridge,  few lights,  radio, and a couple of fans 4000 watts or less should be overkill.   Now if you add on a few items like a furnace fan or air conditioner, well the wattage needs seriously increase.

More watts may seem better, but how much of an emergency are you talking about ?    The lower the wattage, generally, the less fuel consumption.    This could be an issue in an extended crisis.  Batteries have been mentioned.    The could be valuable if you want some electricity available but not run the generator 24/7.

There is also the subject of neighborhood and special needs.  I advised a person to run on a trickle charger, 2 deep cell batteries,  and a cheap inverter. He lived in an upscale neighborhood  and he had a child with autism.  Both would react tobz loud generator. All the child need was the assurance of the lights.  Power outages wouldbe overnight at the most.
 
Eric Hanson
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Regarding THD (Total Harmonic Distortion),

So what I hear is that the magic number for THD is 5% or lower.  I checked specs and the THD for the Generac 5000 XD was 6%, and I would have to think that my gas version is lower.

I have run my house on the generator, including multiple computers, and lots of digital clocks/alarms, etc. and I had exactly no problems with anything electronic.

I don’t exactly know what THD means (is it minute voltage spikes?), but I assume that my computers and electronics are safe with my rather conventional gp5500.

This is information I would personally like to know and might be quite useful to the OP.

Thanks in advance,

Eric
 
John Weiland
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Just my two cents worth as we are on the opposite side of the state from the OP, yet at a similar latitude and possibly similar power requirements.  First, although we use heating oil/forced air for heat (supplemented heavily with a main-floor wood-burner), all other appliances are electric---the kitchen range, water heater, etc.  So we've marked the main breaker box to indicate those loads that should be turned off when running the generator.  The well pump (240V) is a priority for watering animals, although in a pinch we are fortunate enough to have a river across the property to water them as well.   The rural power cooperative actually installed the transfer switch and 4-prong receptacle for a  30A cable ...... and we made sure than any generator to be used will have 120/240 outlet plug to be compatible with this receptacle.

Since you are near Duluth, I would encourage you to check out Northern Tool's offerings on generators---they have a pretty wide range in terms of power and price.  The "NorthStar" line is more expensive and probably longer lasting with Honda engines and perhaps better components.  I went more on the cheap and have a PowerHorse 9000W.  Since we have tractors, I also have one of their PTO-based 7200W models, but may go up to a 10kW model with a newer tractor that was purchased a few years back.  In both the stand-alone and PTO tractor cases, we *have* on occasion put them under stress, but was due to negligence where either an unneeded circuit was not disconnected on the breaker panel or we had the perfect condition of all other circuits coming on a once (well-pump, chest freezer motors, microwave oven, etc....).  In all, these have been more than sufficient, both for house power as well as for remote power on the property for operating electrical tools.  And.....as you noted yourself....the power coops do a pretty amazing job with minimal downtime during power-outages.  So we've used them, but they do sit idle for some time.  For that reason, I've also installed an in-line stop-cock that allows the draining of the gas from the tank for periods when they are not in use.  

Final note on microwave.....I've noticed as well the surge amps that it appears to need, even though technically it is rated not much differently from my toaster for power consumption.  Just something to keep in mind along with other surge loads when making decisions and calculations.
 
Thomas Tipton
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If you have a well pump, you better think big.  I also had an electrician install a backfeed system for my home and I was disappointed to learn that my 6000 starting watt, 5000 running watt gasoline generator was not up to the task of starting my well pump.  Too few amps and you will damage or destroy the pump in attempting to start it.
 
John C Daley
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There is no need to have

rather than have a stinky noisy generator.


The ability to get things quite and run well is available.
Propane as a fuel is a good start.

The lower cost gene's may work for you, but the work harder and fail much sooner than abetter quality unit.
I am currently running a budget 3Kw unit, and looking for a secondhand better quality unit.
 
John F Dean
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Do shop around as time permits.  It is common for people to buy generators, not use them, and then sell at a cut rate price.   I picked up a new 8000 watt Honda after a semi truck loaded with them over turned.   My memory is fuzzy, but I think it was around $300.  

Always consider the trade offs.   As a general rule, smaller uses less fuel.  Of course, bigger runs more equipment.   I am currently shopping for a duel fuel small generator to go with the one above.  The smaller generator should have the advantage of being able to be more easily moved around my property.
 
thomas rubino
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See my early post in this thread, about the predator 3500 inverter gen.
Convert it to propane and your worries are over.
Can't say enough good about this unit.
Quiet , sips fuel, idles down...
This is a honda quality unit at half the cost.
At times we run ours hard and it never fails.
We do buy the extended warranty and trade it in at the end for a new unit.  Cost is the price of the new warranty.

I think they are now offering a larger output in the inverter line but I'm not sure.
 
Michael Fundaro
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In an emergency, what items would you REALLY need to have power?  Fridge?  Freezer?  A few lights which draw minimal power?  Maybe a hot plate or if necessary the stove/oven?  A/C or maybe just fans?  Make a list of what you think you would actually need, look at the labels on each and add up the total watts, and get a generator that provides more than what you need.  If you get a generator that barely meets your needs it will have to run full power and use a lot more fuel.  Buy more than what you need and it will likely run in its "sweet spot" and be more efficient.
You don't need the TV or stereo or all the clocks and all the lights and all the charging devices and the sewing machine and all the other stuff all the time.  If you need a power tool turn off the fridge for an hour.  If you need the A/C it may only be needed at night just to cool things off for bedtime.  Most of what we use in our homes is just wasting power, in an emergency you only need the basics so plan for just a bit over that and your should be good.
 
Michael Qulek
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Ben Zumeta wrote: I have as my solar backup a 25kw diesel generator that came with my off grid place, and have been told (and intuited) that this is a waste of fuel warranting a smaller generator for basic use on cloudy days. I want to minimize my use of diesel, for many obvious reasons. I get a good amount of sun at this new place (probably 250 full solar battery recharge days/yr).



What's the size of your off-grid battery bank?  The charging rate should dictate the size of the generator you need, and that most likely is higher than you think.  A good rule of thumb for traditional lead-acid batteries is charging at 1/8th of C, with C being the amphour capacity of your bank.  So, you would divide C by 8, multiply that by the charging voltage, then include a multiplicative factor for margin of error.  

Let's say you have Trojan L-16 batteries, wired in series to make a 48V system.  They have an amphour capacity of 400Ah, so you'd need 400Ah/8 = 50 charging amps.  Your 48V battery will want charging starting around 50V, so the total power going into the batteries is at least 50A X 50V =2500W of power.  Thinking you don't want to load your generator down more than 1/3rd of capacity, you need 2500W X 3 = 7500W of generator.  So, what you've got is about 3 times bigger than what it needs to be.

One strategy is to wait till your battery gets depleted down to 50%, then start up the generator then, and charge the battery back up to close to full in the shortest possible time.  That might be around 10kWh of power, which the generator will fill up in about 4 hours.  If you plan ahead, you can organize your workday to utilize the rest of the generator's capacity to get as much work as you can done in that 4-hour time frame.  With that strategy, you might only consume between 1-2 gallons of diesel per session.

Keep in mind though that some inverters are very fussy about the QUALITY of power being fed to them.  What I've found myself, is the the generator power quality goes down as the load goes up.  So, your inverter might be most happy getting power from a generator that is only 1/4th to 1/3rd loaded.  I'd keep that generator, but employ the strategy I mention above.

Lastly, I second more solar.  Grid-tie panels are dirt-cheap right now, and I'm getting 1000W for 260$.  My solar is so cheap that I've way overpaneled my system by about 2000W.  I keep from overwhelming my charge controller by facing some of the panels East, most South, and some West.  On bright sunny days, some panels are in direct sun while others are turned 90 degrees, so the maximal amps is not exceeded for my controller.  On cloudy days though, where light is more scattered, all the panels are more or less getting the same weak light at the same time.  That means I can still make 2.5-3.0 kWh of power on rainy days, which is enough get the batteries to full charge if I am conservative.
 
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