# Real wattage output versus rated - Is engine size a good rule of thumb?



## jasonjax (Mar 15, 2020)

I'm in analysis paralysis mode with my generator plans in preparation of this year's and subsequent years hurricane season here in Florida.

It amazes me how so many of these various but similar brand generators can be so wildly all over the map for rated/starting/running etc. wattage output. And then you have to calculate de-rated output for switching to NG.

Is engine displacement the best way to really evaluate a generator's output?


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## iowagold (Jan 22, 2020)

yes and no..
the best way is to have an independent testing group.

mfg's can go either way on numbers..
honda is reserved on the ratings...
most of the Jina gens are inflated numbers.. some are soo fictitious it is not even funny!
and the industrial gens... well they can be any where on the scale!!

and you need your own testing gear for testing on lp or ng...
it all depends on who is setting up the system..
and how in-depth they are going on the details of the install..

the big thing when setting up a system is to do the site survey first..
see what the actual demand is for your site.
I cannot stress the importance on this enough...

it helps you to make good decisions on your generator choice..
so let's say you have a 200 amp breaker panel..
that is average in a modern 3 bedroom house..
just because you have a 200 amp service you may not need that much power to run!!

set up a power chart recorder.
or do a meter setup like this





GENERATOR_CONNECTION


GENERATOR CONNECTION PANEL METER BREAKER INLET



www.poustusa.com




also your utility smart meter system will show your hourly usage so you could do the math..

then you are down to selecting a generator..
choose wisely!
spend your money only once!
will you also be using the generator for camping?
what do you have for storage space when it is not in use...
what fuel supply are you planning on?
do you require wheels on the generator.
do you want full automatic power backup?

all good questions!!
back to the ratings..
find a dealer with a testing load bank..
test the gen set for real world numbers.
peak power is not the same as a 7/24 run output..
the number I like is 50% reserve power.
that way the gen runs with plenty of room to spare.

good smaller load banks start at $6000.00 usd so not just every one has one.
now for real small generators you can use space heaters... stack them up for a load bank.
and have a real good watt meter setup for the load.

yea there are standards they are supposed to use to the bench mark ratings...
that is why consumer reports got a name and a start...
another is UL or underwriters Laboratory.
the ul rating is a good number... pricy to get that rating testing on gear..

you have a good start for making good choices you are in the forums!
always ask questions before purchase!!


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## drmerdp (Apr 10, 2018)

jasonjax said:


> I'm in analysis paralysis mode with my generator plans in preparation of this year's and subsequent years hurricane season here in Florida.
> 
> It amazes me how so many of these various but similar brand generators can be so wildly all over the map for rated/starting/running etc. wattage output. And then you have to calculate de-rated output for switching to NG.
> 
> Is engine displacement the best way to really evaluate a generator's output?


Engine displacement is a good starting point to determine if a given generator is give you their 'balls to the wall' numbers. Essentially a maybe possible but un sustainable wattage output.

Have you done a load calculation chart? Do you own a clamp multimeter for at the very least powering a bunch of things on and checking how many amps you are pulling from the mains?

NG pulls the rated wattage down considerably partially because of its BTU content, but that isnt the whole story. LPG and NG have high octane numbers, considerably higher then gasoline from the pump. Standby generators that run on only LPG or NG have higher compression ratios and make valuable use of the anti detonation properties of LPG/NG. Gasoline generators have lower compression ratios because of gasolines typical 87oct and can not capitalize on LPG/NG's high octane numbers. Most small engine manuals state the low limit for gasoline octane to be 85.

LPG 112octane
NG 130octane!!


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## jasonjax (Mar 15, 2020)

Thanks for the responses.

I have not performed an analysis other than using the widely available calculators for typical household operations.

I have a 200A circuit coming into the house from the meter so I guess I could perform some sort of analysis there using amps*voltage = wattage ?


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## drmerdp (Apr 10, 2018)

I'm not a fan of those calculators. They always point to a giant generator.


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## jasonjax (Mar 15, 2020)

So sheepish moment for me … went out and looked at my meter. It lists actual kw/hour right on the meter. I guess I'll sample that a few times throughout the day and see where it peaks and size to that peak. I have a hard start on my a/c already so hopefully that's the only massive startup load.

Range is gas, dryer is gas, water heater is gas/tankless so only other heavy-hitter I am aware of is the oven which we wouldn't need to use during a power outage.


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## RedOctobyr (Aug 8, 2018)

(Edit: Sorry, looks like we were both writing at the same time)

What things are you trying to run? Especially for the big loads?

Is your hot water and/or heat electric, or gas/oil? How about the stove? Those loads, if electric, will require significantly more capacity.

If you need whole-house AC, what size is the system?

We have gas heat, hot water, and stove. Lights are fluorescent/LED. I can run our essentials on a 2000W-peak generator. Which has very little bearing on the 100A house service. With my load panel, and a larger generator, the biggest load I could consistently apply was somewhere around 4000W.

I'm not proposing you buy something tiny. But not everyone needs a 12,000W monster machine. Don't generalize by your house service. At least make a list of the things you need to run, and if possible, check their amperage draws.


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## drmerdp (Apr 10, 2018)

jasonjax said:


> So sheepish moment for me … went out and looked at my meter. It lists actual kw/hour right on the meter. I guess I'll sample that a few times throughout the day and see where it peaks and size to that peak. I have a hard start on my a/c already so hopefully that's the only massive startup load.
> 
> Range is gas, dryer is gas, water heater is gas/tankless so only other heavy-hitter I am aware of is the oven which we wouldn't need to use during a power outage.


You definitely do not need a huge generator. What size A/C unit do you have?


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## iowagold (Jan 22, 2020)

jasonjax said:


> So sheepish moment for me … went out and looked at my meter. It lists actual kw/hour right on the meter. I guess I'll sample that a few times throughout the day and see where it peaks and size to that peak. I have a hard start on my a/c already so hopefully that's the only massive startup load.
> 
> Range is gas, dryer is gas, water heater is gas/tankless so only other heavy-hitter I am aware of is the oven which we wouldn't need to use during a power outage.


jason switch out the stove oven to gas!!
grin!
then you are pretty much set for a small honda eu series inverter generator!
I would start with an eu2200i. that is enough to run the lights and the electronics for the gas appliances.

now if you are real good with electrical.
not for the novice.
pull the breaker panel cover and use your clamp amp meter on each leg.
L1 and L2 and look at the current demand while all the lights , tv's etc are on.
this will give you a base line power number. if it is less than 1400 total watts use the 2200i gen.


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## jasonjax (Mar 15, 2020)

Yeah, but the A/C for an August/September hurricane extended outage is a must for this Floridian.


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## iowagold (Jan 22, 2020)

soooo what is the current on the tag for the furnace, and for the ac compressor?
or do you have window ac units or mini splits?

the mini splits are cool now as they have an inverter!!
way lower in power consumption


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## jasonjax (Mar 15, 2020)

I know very little about that stuff. It's a 4 ton 14 seer central A/C unit. The tag on the compressor lists 7360 watts @ 230-1P HTR which I assume means heating. I'm sure those numbers are worst case. I do have the hard start installed and my hourly reading at the meter throughout the day today has been around 7460 each time I've checked.


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## drmerdp (Apr 10, 2018)

4 ton ac would have an LRA of about ~120-130amps and RLA of ~20-23amps
The blower motor will have an inrush of about 15amps and operate at about 5amps

So with the A/C drawing 23amps @240v and the blower motor drawing 5amps @120v your at ~6,200watts with the AC alone. Theoretically...

Honestly, I'd ditch the hard start capacitor and go with a soft start capacitor. Soft starts have the benefit of delaying the compressor about 10 seconds so that the total inrush from the compressor and air handler is lower by staggering their starts.

I have a hard start and it helped but only reduced my inrush by 20amps. A soft start will reduce inrush by 50 or more amps. You'll need a generator capable of 8000watts on natural gas. Which puts you at something rated at 10,000watts on gasoline.

With added refrigerant pressures and efficiency losses from high temps I strongly suggest a quality generator with a vtwin engine.

If you look at these northstar generators, they have the same honda GX630 engine but have different ratings due to their alternators. The difference with these generators and the average chinese generator of similar specs is that these are rated to actually maintain their running rating indefinitely. As @iowagold has mentioned, he's load tested a few china gens and they couldnt maintain their running rating for more then a few hours before failing.






NorthStar Portable Generator 10,000 Surge Watts, 8500 Rated Watts, Electric Start, CARB Compliant | Northern Tool


Rugged, newly designed NorthStar® 10,000 Watt Generator features a smaller footprint and the latest Honda V-Twin engine to <b&g...




www.northerntool.com









NorthStar Portable Generator 13,000 Surge Watts, 10,500 Rated Watts, Electric Start, CARB Compliant | Northern Tool


Rugged, newly designed NorthStar® 13,000 Watt Generator features a smaller footprint and the latest Honda V-Twin engine to <b&g...




www.northerntool.com





I appriciate the appeal of having more then one fueling option and portability which is why I didnt get a standby gen.


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## iowagold (Jan 22, 2020)

I have a link for the microair unit 
on this page





HONDA_GENERATOR


HONDA_GENERATOR



www.poustusa.com




yea the soft start units work real good!!
the only thing that is better on lower power demand is the mini split units..


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## jasonjax (Mar 15, 2020)

Thanks for all the info. I'm of the buy once/cry once mindset. And while the 13,000 watt Durostar for $1,400 was my original plan (converted to NG with a U.S. Carburation kit) I think I am leaning towards one of these and just shelling out the cash. It is a little under 4 grand from an authorized local dealer. HPS12000HE-03/A | WINCO


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## drmerdp (Apr 10, 2018)

jasonjax said:


> Thanks for all the info. I'm of the buy once/cry once mindset. And while the 13,000 watt Durostar for $1,400 was my original plan (converted to NG with a U.S. Carburation kit) I think I am leaning towards one of these and just shelling out the cash. It is a little under 4 grand from an authorized local dealer. HPS12000HE-03/A | WINCO


Perfect. Thats a very nice generator.


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## Melson (Dec 8, 2019)

...and it's built in the USA, which is always a win-win AFAIC.


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## iowagold (Jan 22, 2020)

nice gen! that gx will be a good engine. I would change out the spark plugs to iridium ngk asap.

also order several of those 14-60p plugs from them!
they have them listed at 30.00!
those run real money else ware!!
most were 100 bucks and up!

cool spin oil filter! be sure to get a magnet for the spin filter and the drain plug.

it is going to be thirsty on the large GX
and they are loud... so a gen shack is nice.
FUEL CONSUMPTION
GASOLINE 100% LOAD 1.67 GAL/HR or 280 gallons per week
NATURAL GAS 100% LOAD 200 FT³/HR - 204,000 BTU/HR 
LP GAS 100% LOAD 2.2 GAL/HR - 201,304 BTU/HR

for an industrial small gen it looks good!
oh yea down load all the manuals!! get them while you can and burn them to dvd data for backup.


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## exmar (Jun 29, 2015)

Curious about NG during a hurricane or other "event." I've heard or read that sometimes the NG supply is disrupted or shut off for public safety. Assume that isn't the case during a hurricane as opposed to an earthquake? I've been in a couple of hurricanes, one in RI and two in NJ and didn't notice any issues.


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## iowagold (Jan 22, 2020)

yea on the ng..
if you are in an area where trees can be up rooted and floods.
ng may be an issue..
that is why i like the idea of tri fuel...
that way you are covered for the 3 fuels..
we experienced the ng out in marshalltown Iowa 2 years back..
my clients were happy to have tri fuel gen sets!!
some were out of power for over a month in hot humid temps.
they had ng for 4 hours till they turned off the city after a fire blew up a house..
the darn hot water heater had came loose and let ng build up.
and with all the buildings torn up they decided it was best to turn it off till they could inspect the areas 
now they have a better grid system for the ng..
the hospital had no backup power for 2 days!! so that made changes happen fast!!

kinda like the changes they did to our electric grid where i am now..
separation, and smaller cells so they can do better if they have to shut off just parts of the town from remote.
and now we have smart meters they can shut off a house power fast from remote if they have a fire at an location.

next we will have smart gas meters for the same reason..
and they will have smart valves for leak shut down on the grid in small areas.


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