# Whole House Generator Sizing Question......Replaceing a Natural Gas 40KW



## spur0701 (Nov 4, 2014)

Am just starting some research and am looking for adivce.  Here's the situation.  Am buying a new house, it was built in '91, in '00 the second owner installed a Generac 40 KW natural gas whole house generator.  It's wired into the house and suspossedly runs everything including the three different HVACs that heat and cool the house, the only thing not wired in are a few things in the basement, a hot tub, and the pool.  During the house inspection it came up that although the generator will start up it's won't continue running and shuts down.  The current owner had two different contractors come out and they both say the altenator is shot and to replace it is about 10K but the repair is not guranteed; however, they also say that a new 22KW can be installed for about the same price, will do the job, won't use as much fuel, cheaper to run, will be air cooled, and quiter than the old one.....and they tell the current owner that they were sold a bill of good on the 40 KW generator and that it's way to large for what the house needs.   The owner has gotten a quote for a new install of 11K for a 22KW or 18K for 2 22KW.  We are haggling over what kind of credit they are going to give us on this....would I be OK is just going with one 22KW, how should I handle the sizing?


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## Z33 (Nov 4, 2014)

Wow! 40KW is a beast of a genset. As for the sizing of the replacement, no one here can really help you out with that with out some more information.

For starters:

Is the HVAC gas heat or all electric heat pumps 

if electric whats the emergency heat strip size?

What is an average months usage for dead of winter and middle of spring?


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## Jags (Nov 4, 2014)

Holy crap.  40KW?

Reality check...backup generators are just that - backup.  Do you really need to run the home movie theater and the bowling alley during a power outage?  I would start by figuring the base load of the "necessary" items and go from there.  I would venture a guess that 22kw will suffice for the vast majority of homes in the US.


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## moey (Nov 4, 2014)

You didnt mention the size of your house. It could be 8000 sq/ft with 10 kids you may have 400amps coming in we dont know. Generators are about what *YOU* want. I would be shocked if 22kw didnt handle what you need. 

Do you really need to run your welder when your in the hot tub? Thats meant to be funny...


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## KB007 (Nov 4, 2014)

Go through the house and figure out what you want to run in the event of a power outage, ie what's your tolerance level.  Really - do you need all the hvac units?  You're on here so you probably have some wood heat   Add 'em all up, then figure out if you need them all on at the same time or if you can live with some on/off action at the panel to balance your load.  Once you figure that out, you'll have a decent idea of what you need / want.

Even 22KW is a pretty big home backup generator.  Just remember if you're negotiating to include the cost of some re-wiring if you are changing which genny you're going to use. 

What's your oven run on - gas or electric?  What's your water heater - Gas or electric?  Do you really need the electric clothes dryer in an emergency?  Do you have a well pump or city water? Sump pumps?  What else is in your house that needs 40 KW?  Grow op?  Greenhouse with rare orchids?  Petting zoo with Hyacinth Macaws? (believe it or not, one of the reasons I got our first generator was to make sure we could provide heat for a number of macaws we had at the time...)


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## seige101 (Nov 4, 2014)

Here is another idea. Contact a motor repair company and get a price to rewind the generator. Like other have suggested 40kw may be grossly oversized. How big is your house what is your heating systems and what would you like powered in an outtage situation?


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## Retired Guy (Nov 4, 2014)

Post your question here: http://www.zillerelectric.com/forums/index.php. Be ready with your serial number and model number. The site seems slow/down right now but is normally on 24/7


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## fbelec (Nov 4, 2014)

i had the same question asked of me from one of my customers. they had a well and sewage ejector and 4 geothermal heat pumps the largest heat pump was 5 tons. never mind the 5 ton startup current was 125 amps (30,000) watts and the emergency heat on it took a 100 amp breaker about 66 amps running. and the 3, 2, and 1.5 ton other units that would try running also. they didn't like the $9,000 dollar bill for installation of a 22,000 watt generator. also that generator would only run a week on the 1000 gal propane tank. i sized it at 22,000 watts because the last two dummys were trying to tell them that a 14,000 watt generator would work. they didn't want to hear about the a larger generator's fuel use.
    my point is that 40,000 watt generator is what is needed for that house and trying to run a smaller one would end up with the generator shutting down on overspeed when one of the largest heat pump would try to run. and also the same thing would happen sooner with the smaller one installed because of trying to run to big of a load. the best idea and probably the cheapest would be what was said earlier in this thread is to pull the alternator and have it rebuilt. if you like the house and can live with not running the the heat pumps and run wood stoves then have them give you a check for the 22,000 watt unit and have someone come in and resize and put a smaller unit in. if i remember right the 22,000 watt generator took the same as a 160,000 btu boiler running non stop so gas bill can be sizable for a week long power outage.

good luck
frank


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## maple1 (Nov 5, 2014)

Does the power really go out that much in Maryland?

I have a 3kw portable inverter genny to do us. The primary factor for me in an extended power outage isn't how much juice can I make, but how much fuel will I burn. We can keep warm with no juice with the boiler setup, have a BBQ we can cook stuff on if needed, keep a barrel under the HRV that catches condensate the rest of the year that I can flush the toilets with - so the 3kw keeps us in luxury.

I think I would go along with whatever the homeowner will do to get the generator setup running again - and then sell it & get something a LOT smaller & pocket the difference. I see you already have a woodstove, so you should have the main thing needed in an outage (heat) covered.


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## firefighterjake (Nov 5, 2014)

We're the same as Maple . . . a 3,000 Kw generator. With the woodstove in operation and a gas stove that we can use to cook with (well the stove top at least -- not to mention the propane gas grill), the generator is only put into service to power up the refrigerator, freezer and my wife switches back and forth with the TV and computer.


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## Adios Pantalones (Nov 5, 2014)

Agree with comments above. My backup was sized, largely, by the startup power for my well.


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## spur0701 (Nov 5, 2014)

Thanks for the replies to date, additional information: Power in that area goes out frequently I'm told, hence the reason why they had it installed.  It is a big house, about 9,000 square feet, 5 beds, 7.5 baths, 5 gas fireplaces (some have blowers), and 1 wood fireplace (am taking my Quad 5100i with me if i can).  Heat (3 zones) and hot water heaters (2 at 75 gals) are all natural gas.  Water and septic sewer are city, there is a sump pump.  The cook top is an induction one, not gas, not sure how much that draws.  I know I need  do a walk through and calculate what we need, what we want, and what would be nice but think we'll have to wait till we move in before we do that.......my sense is that 22KW might do it.  From the replies so far I'm looking at negotiating the best credit than I can from the seller since the 40KW is essentially dead and then after we take possession see if the 40KW can be repaired or install new.  The quote the seller has now is 11K for one 22KW (which is 5k for the genset, 4k install, and 2k misc) or 18k for 2 22KWs.  The big issue now is negotiating the credit to replace the the 40KW, since I'm not 100% sure a 22KW will do it I'm leaning toward splitting the differece between the 1 and 2 22KW quotes and ask for 14 or 15k.


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## jebatty (Nov 6, 2014)

For me the primary factor in choosing a generator was a cost/risk analysis with a small adjustment for convenience. In my area power outages are usually brief (a few hours) and not very common, but an outage of several days with lots of trees down and roads blocked must be considered from a winter storm when temperatures could be well below 0F and plumbing freeze up would be a real consideration; or a similar outage with a severe summer storm and a need to keep refrigeration/freezer operating. 

Winter heating and freeze up not an issue, heat with a wood stove in the living room with no need for electricity; also have a supplemental LP wall heater with a millivolt thermostat and no need for electricity. Summer need same as winter, can get by without AC during an outage. 

Electricity need: some lights, microwave, well pump, occasional power for the refrigerator and freezer; and for sanity power for the DSL phone/data line, computer, television/entertainment and keeping the 5V devices charged. All of our lighting is LED now, so the transfer switch now powers lighting, the microwave circuit, refrigeration/freezer circuits, well pump (240V), and the electronics circuit. Total power needed: about 5500 surge watts (well pump) was the determining factor, which also resulted in about 3500 continuous watts output. Except when the well pump kicks on and surges from the refrigerator/freezer, actual continuous power demand is under  1000 watts. 

Result: risk is low, emergency power need is small, and I bought an inexpensive 120/240V gasoline generator with plug-in output to a transfer switch powering needed circuits. I also keep 25-30 gallons of gasoline on hand in 5 gallon containers which I rotate by filling the gas tank on the car as needed, always fresh gas.


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## maple1 (Nov 6, 2014)

spur0701 said:


> Thanks for the replies to date, additional information: Power in that area goes out frequently I'm told, hence the reason why they had it installed.  It is a big house, about 9,000 square feet, 5 beds, 7.5 baths, 5 gas fireplaces (some have blowers), and 1 wood fireplace (am taking my Quad 5100i with me if i can).  Heat (3 zones) and hot water heaters (2 at 75 gals) are all natural gas.  Water and septic sewer are city, there is a sump pump.  The cook top is an induction one, not gas, not sure how much that draws.  I know I need  do a walk through and calculate what we need, what we want, and what would be nice but think we'll have to wait till we move in before we do that.......my sense is that 22KW might do it.  From the replies so far I'm looking at negotiating the best credit than I can from the seller since the 40KW is essentially dead and then after we take possession see if the 40KW can be repaired or install new.  The quote the seller has now is 11K for one 22KW (which is 5k for the genset, 4k install, and 2k misc) or 18k for 2 22KWs.  The big issue now is negotiating the credit to replace the the 40KW, since I'm not 100% sure a 22KW will do it I'm leaning toward splitting the differece between the 1 and 2 22KW quotes and ask for 14 or 15k.


 
The only things I am seeing in there that I would term 'essential' in needing genny power in a power outage, are the sump pump, and blowers to circulate the NG heat. Water & septic are covered by city supply. Cooking could be covered by a microwave & BBQ that has a side burner. Anything else, like lights, TV, computers etc. shouldn't consume much as long as the whole house isn't lit up. What I would do for sure after you get through the credit issues & sale, is put an Effergy on your mains so you can really see how much power is being used in typical 'every day' conditions - then size a generator to that.

How much are the monthy power bills for this place, anyway? Even if the average load was 10kw (1/4 the size of the current generator), that would amount to like $1400/mo at 0.20/kwh.


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## jharkin (Nov 6, 2014)

firefighterjake said:


> We're the same as Maple . . . a 3,000 Kw generator. With the woodstove in operation and a gas stove that we can use to cook with (well the stove top at least -- not to mention the propane gas grill), the generator is only put into service to power up the refrigerator, freezer and my wife switches back and forth with the TV and computer.



+3

I also have a 3200 watt unit.  I got this to have enough juice to run the sump pump and the septic pump through a long outage in a severe rain/flooding event.  Bonus is that I have enough juice to occasional cycle the fridge and keep food cold or run the water heater for a hot shower.

Without those two pumps I could probably survive for weeks on one of those $89 harbor freight deals.  heat by wood and cooking by gas.


Oh and being close to a major city, I have not once actually needed it since that first outage incident in 2010 flooding rains that prompted me to buy it.


Its your money of course, but I agree with the advice to consider what you really _*need*_ before committing so much coin...


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## jharkin (Nov 6, 2014)

OK  i missed the 9000 sq feet part.... maybe have your own personal nuclear plant installed    (haha couldn't resist)

But seriously... you say the power goes out frequently... but are they long outages?  Maryland is a pretty densely populated, relatively urban state right?.  If they have outages a lot but they dont typically last more than a couple hours you can probably get by just waiting it out most of the time.  Its a very isolated rural area prone to outages lasting days (I think Bart deals with that for example) that I'd want the dedicated backup.


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## maple1 (Nov 6, 2014)

I'll add to my last post - more essentials to add not mentioned would be freezer & fridge.

And I still have a hard time comprehending the power would be out a whole lot if you are in a city? But I could certainly be off on that.


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## moey (Nov 6, 2014)

22kw would be fine. You don't have electric heat and your hot water heaters are not electric. But if its part of negotiating a sale shoot for the moon. 

The only items you need to be concerned with are the sump and stove and dryer if its electric. Thats less then 10kw right there. That leaves 12k for everything else and to handle a startup surge. If your even close to touching 22kw you need to remove the heating lamps throughout the house and replace with LED's. Oh and dont run the welder and the hot tub.


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## briansol (Nov 6, 2014)

Who does laundry in an emergency situation?  lol

needs VS wants


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## spur0701 (Dec 7, 2014)

Update:  Hi guys......just an update and looking for more advice.  Negotiated a fair credit for this generator problem and closed on the house.  We have a contractor doing some mods and aren't planning on moving in till he is done, which is Jan or Feb.  

More details on the generator, it's a commercial 40KW that was manufactured in 01 and installed in 02.   I had one of the contractors that the seller was dealing with trying to get it repaired out for a chat and they say that it's not the alternator that's bad but the generator itself needs to be rewound and they can't find anyone to do it and Generac doesn't have parts for this generator any longer.  Had another one come out that had given her a quote for replacing it........Both say the genset is so big compared to the house that it's a joke, it's so big it could run two houses this size, a 20 or 22KW residential will work fine, and that the existing genset has no value and is going to cost 1 or 2K to demo and remove because it is so big, they need a front end loader to get it out from behind the house.  

I did some google searches and from the results think rewinding it might be outside the capability of a shop that would rewind alternators and though I found some companies that perform that service it looks like they are all commercial and/or outside the US.  So at this point I'm not sure repair of the existing genset is practical so we are looking at possibly replacing it.

The first contractor says the existing switch will work fine, the second contractor says it needs a new switch.  The prices from both are pretty much the same for doing a new install plus removing the old genset....12K to 13K....so the existing stuff from the existing genset doesn't seem to be giving me any break on price.

Northern Tool has residential 22KW Generacs for about $4500 with free delivery.  My current neighbor (who is a VP of a construction company and has a 16K genset on his house) says my best course of action should be to find someone to demo the current one and then an electrician to simply hook up the new one in it's place using the existing switch and wiring config inside the house.....the only problem might be that the current natural gas hookup may need to be down sized from the 40KW for the 22 KW.  Anyone have any thoughts on that course of action?

I currently live in rural redneck southern Maryland and we have a electric coop (SMECO) that seems pretty good, we rarely lose power.  This new place is in Great Falls, VA....which is in northern VA and close to DC.....but it's heavily wooded and power is out a lot I am told.  I need to do research on how often it really goes out and was thinking of just putting in a battery backup for the sump pump and forgetting the backup generator but the wife isn't really digging that idea and wants the genset......


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## maple1 (Dec 7, 2014)

Has any figuring or monitoring been done in the last month on how much electricity the house actually uses?

Or I should say needs?

Re-reading, I'm still not seeing the need for anything approaching the size of even the 22kw unit.

If the sump pump is heavily depended on - a combination of battery backup for it & a generator might be something to think about too, just to cover for the chance of the genny not firing up & taking over right away when the power does go out.


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## fbelec (Dec 8, 2014)

if there is a electric motor rebuild company around you somewhere that is who you want to talk to. i've see the motor shops up here rebuilding motors that i could walk thru. the oly thing they don't touch are the small motors that are found in bathroom fans. when we speak of the alternator we mean the generator side of the unit and not the alternator or generator that is keeping the starting battery charged. thinking about the 12 to 13 k they want to charge you to as we call it (pull and push) a new unit in place sounds kind of high. they don't have to do any digging, wiring the transfer switch, running the separate line to keep the battery charged, natural gas line, it's all there now. the only thing you have to worry about is if the transfer switch will mate with the new generator. it sounds simple but some transfer switches are pretty fancy and might not work. also i find it kind of hard to believe that the alternator is burnt out and people are telling you that you could use a smaller generator. it doesn't sound right to me. if the load was not a big load on the 40 k unit why would the alternator go bad?  it might be well worth a call to generac themselves and tell them your load and see what they have to say about the size. and before i forget again i reread your posts and i'm not sure what your heat is. the first post sounded like heat pumps but the second post sounded like boiler with 3 zones. what is your heat? if it is not heat pumps then use the smaller generator. if your heat is a hydroair get the smaller generator. with this type of heating you only need the blower fan on each hvac unit for heat. if you have a big boiler or twin boilers with about 3 to 400,000 btu then most likely you have a hydroair and that is why you only have three zones one for each hvac unit. good luck and let us know. 

frank


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## woodgeek (Dec 8, 2014)

Downsizing the gas line is not an issue....just a fitting.  Upsizing the line...that is another issue.

Sounds like you should shop for a 15 to 22 kW solution and have a happy wife.

My solution: a $100 700W genset and some extension cords run all critical systems.

If the induction was a killer (unlikely) you could buy a nice propane camp stove and set it on the cooktop as needed.  We have an electric range, and just use a 2 burner propane stove when the power is out.


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## ironpony (Dec 8, 2014)

is the generator actually dead? or does the circuit need excited? when generators sit for a while they can "die" do a google search it might be this simple. I have done it on smaller generators that were supposedly dead. cost = free.

back up generators have gotten totally out of hand IMO. the money spent to maybe use it a couple days a year is insane. even if it was a week, really? another way to separate you from your money. 5 kw will run everything you "need" in a house. I run a 6.5kw.

yes, I own four 25kw diesel generators and I do not even bother to tow one home in an outage.


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## seige101 (Dec 8, 2014)

You can always look into putting a new generator head on the unit you have http://www.dieselgeneratorsmiami.co...e-pi144-1-phase-alternator-generator-end.html


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## Bret Chase (Dec 9, 2014)

ironpony said:


> yes, I own four 25kw diesel generators and I do not even bother to tow one home in an outage.



I've got a gas powered welder that puts out 9500 continuous... I don't bring it home either... What I generally do is just plug my 250w inverter into our Suburban... and use it to power up the modem, router.... and charge the laptops. The rest.... I run the woodstove if we're cold.... use a lighter to light the top burners of my gas stove.. and if I'm hell bent on baking something (like thanksgiving of this year)... drag an extension cord 250ft to my brother's house (who ALWAYS runs a generator).... to power the 300w hot surface ignitor in my oven.

And when it gets dark... light the kerosene lamps we have as backups. 

The thought of needing a 22k or 40k genset is just insane to me....


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## ironpony (Dec 9, 2014)

so I am not crazy..........


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## velvetfoot (Dec 9, 2014)

I guess you aren't on a well.


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## Bret Chase (Dec 9, 2014)

velvetfoot said:


> I guess you aren't on a well.



I am actually...  I keep 100 gals of water as backup.


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## velvetfoot (Dec 9, 2014)

Bret Chase said:


> I am actually...  I keep 100 gals of water as backup.


That's interesting.  I should do that too.  Can I ask how you store the water?


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## ironpony (Dec 9, 2014)

velvetfoot said:


> I guess you aren't on a well.





I am and a 5000 watt generator will easily run a deep well. 240v 20 amp draw rated actually less.


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## velvetfoot (Dec 9, 2014)

A 5kw generator didn't do it for my well pump.  I had to get a bigger generator.  Having some stored water would stretch out the time before I'd have to bring that one out.


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## Bret Chase (Dec 9, 2014)

velvetfoot said:


> That's interesting.  I should do that too.  Can I ask how you store the water?



gallon milk jugs and food grade 5 gal buckets... and I rotate and replenish the water... i.e. grab a gallon of water... fill the dog's dishe's... or the pot on the woodstove... refill


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## Bret Chase (Dec 9, 2014)

ironpony said:


> I am and a 5000 watt generator will easily run a deep well. 240v 20 amp draw rated actually less.



My 1hp jet pump is on a 240v 15a breaker.


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## velvetfoot (Dec 9, 2014)

Thanks.  I might give it a whirl.


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## maple1 (Dec 9, 2014)

I think I mentioned earlier what I do for water.

I keep a 25 gallon plastic barrell under our Venmar drain. That, and what's in the tanks of our 3 toilets, handles toilet flushes.

Between the water pitcher in the fridge, and what the cushion tank holds - that holds us for drinking water. A bigger cushion tank would pay double dividends here - more water in reserve when the power goes out, and less pump starts/stops the rest of the time (save some on the power bill). Ours is only 6 gallons I think. Would like to have a bigger one or add another, just another one of those things I haven't gotten around to yet. (That's a looonngggg list).

That will get us by a few days.


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## ironpony (Dec 9, 2014)

I manually transfer my generator to my main panel, turn of the heat pump and HW tank, everything else runs fine. the odds of to many things, frig, freezer kicking on at the same time is slim.


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## pdf27 (Jan 5, 2015)

Any space for solar? $15k goes a long way to paying for a big solar array with a Sunny Island unit to keep it operating during blackouts. That can also take batteries to provide backup power overnight, and unlike a generator that would give you some payback in reduced electricity bills (which are presumably huge on a 9,000 sq ft house! The downside is that you aren't going to get the full 22kW out of a system like this without much higher outlay, but on the plus side you'll never run out of fuel.


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## Badfish740 (Jan 6, 2015)

spur0701 said:


> The owner has gotten a quote for a new install of 11K for a 22KW or 18K for 2 22KW.  We are haggling over what kind of credit they are going to give us on this....would I be OK is just going with one 22KW, how should I handle the sizing?



How about looking into a diesel 22kW?  It will run slower, quieter, and most importantly, live longer, than a natural gas generator of the same size/output.


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## razerface (Jan 6, 2015)

I am getting ready to remove a 25kw from my house. It will be replaced by a Miller engine fired welder/ generator. I need the welder for portable welding around the 40 acres, and it will generate 8kw when electric goes out.

I won't have auto switch since outage may be short, and my house will hold heat for many hours


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## fbelec (Jan 8, 2015)

spur0701 how are you making out? did you do anything with it?


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