# Whole home generator size?



## Hoodtfcr (Sep 4, 2020)

So I am trying to decide what size generator I need for whole home backup. I work for the power company and can pull the load data for my meter for the last 12 months. The peak KVA I have pulled at any one time during a day is 13.09. So if I convert this to KW with a power factor of 1 it’s 13KW. So if I install a 19.5KW generator this should backup my whole house. Does this seem correct to you guys? I can’t seem to figure out why the generator sales man is trying to sell me something different.


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## ToolLover (Jan 13, 2020)

That 13.09 KW load does not tell what the start ramp load is.
Is the house total electric? Is the AC big?
There are a lot of factors to making a decision about your needs.
You can manage loads and get by with less, but then when the little lady is home alone, load management just doesn't work too well.
I do not know of a 19.5 KW generator that is available
Take the leap and install 20 - 24KW and get it over with.
I prefer an 1800 rpm unit over the loud 3600 rpm generators.
Better life in the 1800's too.
My pick at this time:





LP/Natural Gas Generators (Stationary/Standby) | Powerhouse Diesel Generators


LP/Natural Gas Generators (Stationary/Standby) | Powerhouse Diesel Generators




www.pdgenerators.com




Of course the cost adds up with the better generators.


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## iowagold (Jan 22, 2020)

Hoodtfcr said:


> So I am trying to decide what size generator I need for whole home backup. I work for the power company and can pull the load data for my meter for the last 12 months. The peak KVA I have pulled at any one time during a day is 13.09. So if I convert this to KW with a power factor of 1 it’s 13KW. So if I install a 19.5KW generator this should backup my whole house. Does this seem correct to you guys? I can’t seem to figure out why the generator sales man is trying to sell me something different.


the best thing to do is take inventory of what you are running or need to run.
make a spread sheet with start amps and run amps as well as the voltage (125 ac or 240 ac)

i prefer to get a data recorder from the utility company that should be easy for you!!
set that up for a week to get real time numbers. make sure to set it up for L1 and L2 data.

for me i always say you need 50% more than your max demand.
so if you are at the 13 kw i would look at 24kw to 26kw but get one that is slow rpm and real good on fuel!
there are a few out there now with efi on lp and ng.

i prefer at least a bi fuel or a tri fuel setup so you can have a choice of fuels...
diesel runs second low cost per hour from natural gas at least here in the midwest.

for me, well i can run the house on 1000 watts in winter!
and summer is more with ac units...
so plan your system... 
maybe find out what is drawing so much power!!
then choose a gen set after you get the large demand issue under control.

right now I am working on kicking the energy hog of an electric water heater to the curb!!
it is a work in progress for sept 2020!
also doing a water recirculate system with new pex for the whole house!!

it will be cool when it is all done!!
and i can run the house temps lower during the cold months!!
get back to my 66 to 67 deg house i like!!
lol!


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## drmerdp (Apr 10, 2018)

Hoodtfcr said:


> So I am trying to decide what size generator I need for whole home backup. I work for the power company and can pull the load data for my meter for the last 12 months. The peak KVA I have pulled at any one time during a day is 13.09. So if I convert this to KW with a power factor of 1 it’s 13KW. So if I install a 19.5KW generator this should backup my whole house. Does this seem correct to you guys? I can’t seem to figure out why the generator sales man is trying to sell me something different.


20KW should be plenty. Thats great that you have specific numbers to work with.

Do you have natural gas or propane?
Are your large appliances gas or electric? (water heater, furnace/boiler, range/oven, clothes dryer.
Do you have central air, how many units and what tonnage?


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## iowagold (Jan 22, 2020)

and if you have central air have to done the soft start unit yet?
you will want that for a gen system.

if you have natural gas or lp a good water heater to run on those fuels save bucks and demand.
as well as stove and dryer..
those are the heavy hitters for electric in the houses these days!!

and you can run both fuels for water heater..
i am doing a system that is both electric and natural gas for water.
that way i have a bi fuel system for storm outage.
a lower wattage electric heater with a recirculate pump for the no freeze during cold winter!!
it will be a cool system when it is all done!!


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## RASelkirk (Sep 18, 2020)

Can one go by the electric bill's average kW hr per day? Just curious, my bill for August here in SETX (hottest month) shows 77.6 which works out to 3200 watts per hr. Pretty sure I won't be running whole-house on that with a 4T A/C unit and electric oven!

Russ


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## drmerdp (Apr 10, 2018)

Average kW doesnt consider starting wattage of induction loads. Not the best option. 

You can simply get a 20-24kw unit which is probably way to big, especial if you have only one central air unit and non electric heat, stove, or hot water.

Or do a smaller unit thats probably fine, and add load shedding to cover the potential of over loading the generator.


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## iowagold (Jan 22, 2020)

and to help with inrush on the compressor use a soft start unit
that micro air unit works!
you need one on each air con unit.

you might look at tri fuel as well...
a smaller gen setup might work..
and look in to a gas range as well as water heater as gas.
you can always use a natural gas grill or liquid propane grill for outages.

what is you area of the country RA?
cali as well as florida and Iowa bunch can tell you one fuel is not the way to go..
darn trees get in the gas lines and if they fall over the tear up the gas lines..
and you can be weeks with out natural gas..
that happened in the 08/10/2020 storm here in Iowa.

you can parallel 2 eu7000is gens to get larger power and run it as tri fuel.
then you can fall back to one gen set for winter months.

or upgrade the old air con unit to a state of the art inverter ac unit to save power demand.
some have gone to the mini split units!


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## lillianlambert (10 mo ago)

In my house, I have a Generac 15kw that was already installed with it, running on propane (a 250gal tank that I upgraded), and I pay around $400 a year for a service plan. I have 3 heat pumps that are 12kw each, so I turn those off when the power goes out. The generator keeps on the lights, internet, kitchen, and basement appliances. I can run one heat pump for air conditioning off it. I have a lightning house number plaque that also works with the power from the generator.


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## sdowney717 (12 mo ago)

My 8000 watt, 13,500 watt surge Troybuilt starts my 3.5 ton heat pump just fine.
I wired it into my house panel using an Interlock Kit made by Square-D

We almost never loose power, longest time ever was 10 days during hurricane Isabel. That was very unusual.
Bigger gens are going to use more gas, cost more overall to buy, operate and service, for what amount of time period is it worth that.
My goal at first was simply to run a gen as needed knowing the power would return, but finding out it could run my heat pump was a bonus which I like. It is really in the surge capacity of a gen, what kind of loads it can start.

I paid $50 for this one, from Craigslist, carb had filled engine with gas, and seller thought it was ruined, but it was fine. just needed carb cleaned.


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## Robh (Aug 19, 2019)

Hoodtfcr said:


> So I am trying to decide what size generator I need for whole home backup.


People are quoting hard numbers here, but remember that'll cost you a pretty penny to buy and run (fuel!) if you only need 25% of that. You have to ask yourself the soft figures.
How often does power go out?
How long each time?
What can you get away with during that outage period, or do you need your A/C and electric dryer?

For the most part, I can run my whole house in the winter on a 2200W portable generator. I can't run the dryer, A/C, or hair dryer, but can manage the microwave and coffee maker if I'm careful. If I bump it up to a 6000W 120/240 generator, I could do everything but the first two, all while enjoying a tremendous savings in fuel costs.

There is a reason why this forum has many posts about people running portable inverter generators for their "whole house". It's that mentality. Now I'm not saying there is anything wrong with a 20KW generator if you want it, but there are more factors involved in making a decision than looking at the peak load and picking something just above it. For a short outage, the mode is sometimes more useful than mean or median.


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## zz28zz (Nov 24, 2021)

The max constant load is pretty straight forward and sounds like you have that number. Max continous load should be between 50% and 80% of genset's max continous load rating.
The other big concern is start-up current for the big users like AC and any pumps (pool, well, septic etc) you may want to include. They pull roughly 5X their running amps during start-up. That 20A A/C compressor will pull close to 100A for a second or two when it starts. The newer inverter type compressors will probably draw significantly less at start-up.
A "soft start" on the compressor will help too as prev mentioned but they can be pricy to purchase and install.
Some generators handle brief over-current conditions better than others. The gens with a larger rotating mass typically handle brief heavy loads better. Calculating the high in-rush loads can be tricky. Simplest way to determine min gen size would be to rent a genset the size you think is best and give it a try. If it works, buy that gen or larger.

There are a ton of factors involved in making your decision other than min size including:
Type of fuel. If gasoline genset is converted to propane, subtract 10% from genset rated power. If switching from gasoline to natural gas, subtract 20%
Price of fuel
Availability of fuel in an emergency
Fuel storage considerations
Distance from fuel source (propane/ nat gas)
Longest expected power outage
How often you loose power
Genset distance from elec panel ( wire is getting expensive)
Your budget
If your doing it yourself
Permanent or temporary install
Load-shedding options incorporated or not
Allowable noise levels for day and night
Where it will be stored (outside or in garage)
Where it will be used (need sturdy/level surface out of the rain and far enough from doors/windows for the exhaust to not kill ya)
Complexity/expense of install. Soft starts, load shedding, additional breaker boxes, running new wiring/circuits, etc all add up quickly and add to the list of things that can break.
Re-sale value if you decide you don't want it any more.


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## JVazquez53 (Jul 8, 2021)

At least for me, a Honda EU7000is runs my whole house without any issues. I do "play" with the watts. I just dont turn on everything at once. By running one 24,000 BTU inverter mini split AC, all the lights (LED) couple of TVs, two refrigerators, the generator display shows at worst case 1900-2200 watt consumption. If I need to turn the water heater, I turn off the AC, same for washer and dryer (gas)..


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## JVazquez53 (Jul 8, 2021)

zz28zz said:


> The max constant load is pretty straight forward and sounds like you have that number. Max continous load should be between 50% and 80% of genset's max continous load rating.
> The other big concern is start-up current for the big users like AC and any pumps (pool, well, septic etc) you may want to include. They pull roughly 5X their running amps during start-up. That 20A A/C compressor will pull close to 100A for a second or two when it starts. The newer inverter type compressors will probably draw significantly less at start-up.
> A "soft start" on the compressor will help too as prev mentioned but they can be pricy to purchase and install.
> Some generators handle brief over-current conditions better than others. The gens with a larger rotating mass typically handle brief heavy loads better. Calculating the high in-rush loads can be tricky. Simplest way to determine min gen size would be to rent a genset the size you think is best and give it a try. If it works, buy that gen or larger.
> ...


For fuel, mine are all gasoline, one is dual fuel
Distance from fuel: 30 feet
Longest expected power outage: last big one, 4 months
How often we lose power: every couple of months
Genset distance from panel: 100 feet, the wiring job was pricey but worth it ($1,500)
Shed: Permanent,











































solid concrete with steel door, hurricane proof for the two inverters, the third one(noisy one) it's chained on the roof but protected from the rain/wind
Noise levels: low, thanks to the shed plus the generators are quiet, 52Db
Resale value: high, Hondas are easy to sale at a good price, The Firman I'm not worry about that one, that's was the cheap one.


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## pipe (Jun 27, 2021)

JVazquez53 said:


> If I need to turn the water heater, I turn off...


If you have a standard 4,500watt/240volt water heater, this simple switch will change it to 1,125watt/120volt and there is no need to turn off hot water

There is no downside. Hot water simply heats up slower but reaches same setpoint eventually.

As an aside, just installed an Inverter Clothes Washer yesterday. Very little power use as long as I do not use it's internal heating system. No belts, just a DC motor to drive it.


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## GenKnot (Dec 20, 2021)

pipe said:


> If you have a standard 4,500watt/240volt water heater, this simple switch will change it to 1,125watt/120volt and there is no need to turn off hot water


That's pretty slick.








How to Toggle Electric Water Heater Between 120V and 240V?


How to wire electric water heater for both 120V and 240V. Toggle Water Heater between 120V and 240V using DPDT and 3-Way Switches. 120 & 240V Heater Wiring




www.electricaltechnology.org




I'm glad I am on NG. I have to remember to plug in the ignition transformer because it doesn't have a standing pilot though.


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## sledman8002002 (Jan 28, 2021)

GenKnot said:


> That's pretty slick.


I agree, I like that option.


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## Nebrasky (Dec 30, 2020)

pipe said:


> If you have a standard 4,500watt/240volt water heater, this simple switch will change it to 1,125watt/120volt and there is no need to turn off hot water
> 
> There is no downside. Hot water simply heats up slower but reaches same setpoint eventually.


Question for anyone: Does this switch setup actually reduce the amperage draw on both legs, or only on one leg? 

In other words, normal operation is approximately 19 amps on both legs. 1,125 watt/120 would be 10 amps on one leg. 
Is that correct? It makes the water heater an unbalanced load, but a little over half the load, even on that leg?


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## GenKnot (Dec 20, 2021)

Nebrasky said:


> Does this switch setup actually reduce the amperage draw on both legs, or only on one leg?


Switching the water heater to 120V using the suggested 3-way switch would cause only one leg to be used. The other leg would have 0 amps. So yes, it makes the water heater an unbalanced load.


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## Nebrasky (Dec 30, 2020)

GenKnot said:


> Switching the water heater to 120V using the suggested 3-way switch would cause only one leg to be used. The other leg would have 0 amps. So yes, it makes the water heater an unbalanced load.


I understand that it makes it an unbalanced load. My question is, what happens to the load on the leg that's still being used? Does it actually reduce the amperage draw on that leg?

To rephrase it, in normal operation we have 19 amps on both legs. In switched operation, do we have 10 on one and zero on the other, or 19 on one and zero on the other?


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## GenKnot (Dec 20, 2021)

Nebrasky said:


> Does it actually reduce the amperage draw on that leg?


No because the resistance of the heater element remains the same. The current would still be about 10A @ 120V, but the power would be 1/4 of the original.


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## Colt Carson (6 mo ago)

JVazquez53 said:


> For fuel, mine are all gasoline, one is dual fuel
> Distance from fuel: 30 feet
> Longest expected power outage: last big one, 4 months
> How often we lose power: every couple of months
> ...


I gotta ask… what happened that you lost power for 4 months?


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## tabora (Sep 6, 2018)

Colt Carson said:


> I gotta ask… what happened that you lost power for 4 months?


He's in Puerto Rico... There was this little disturbance called Hurricane Maria.


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## sdowney717 (12 mo ago)

GenKnot said:


> No because the resistance of the heater element remains the same. The current would still be about 10A @ 120V, but the power would be 1/4 of the original.


Ok, but are you really saving any power seeing it takes a lot longer to heat up the water, it is going to be running longer.
This is just a way to save power use when using a power limited generator as you are not saving any money when on utility power. And you don't get something for nothing,


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## GenKnot (Dec 20, 2021)

sdowney717 said:


> Ok, but are you really saving any power seeing it takes a lot longer to heat up the water, it is going to be running longer.
> This is just a way to save power use when using a power limited generator as you are not saving any money when on utility power. And you don't get something for nothing,


Who said anything about saving power? The power to the element(s) with 120V is 1/4 of what is at 240V. It will take about 4 times longer for the water heater to recover.


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## sdowney717 (12 mo ago)

GenKnot said:


> Who said anything about saving power? The power to the element(s) with 120V is 1/4 of what is at 240V. It will take about 4 times longer for the water heater to recover.


That is the whole point of the thread, and the mention of the switch.
You don't see that your saving power but using up time.
So who said anything about saving power?
why did you bother commenting, people running generators are trying to use less power, so then saving power is what this thread is about.


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## OrlyP (Oct 19, 2021)

I think we've fallen into a crack. "Saving power" is not the best statement to use in this context. Instead, what you are trying to achieve is lowering the operating current so it doesn't use up your generator current "budget".

If you feed a heating element (purely resistive) that is half its rated voltage, the current will be halved as well and the resulting output power will be down to 25%. That's Ohm's Law.

From this formula: *P = V² / R*

Where:
P = Power in Watts
V = Input Voltage
R = Load resistance in Ohms

Let's say that the Voltage is 240V while the heater element is 50 Ohms. That will give us:

P = 240² / 50
P = 1,152 W

If we halved the voltage:

P = 120² / 50
P = 288 W

Another formula is *I = V / R*

Where:
I = Current in Amperes

So if V = 240V and R = 50 Ohms (same as above), then

I = 240 / 50
I = 4.8 A

If input voltage is reduced to 120V, then

I = 120 / 50
I = 2.4 A

So at the end of the day, with the voltage input to the heater halved, it will take at least 4x as long to heat it up to the thermostat setting.... maybe even longer due to system losses (insulation losses).

With the voltage halved, it will also be pulling half of its rated current. However, you are technically _consuming_ more power over the longer length of time it takes to heat it (ie. higher Watt-Hours) because you're also giving time for the heat that's already in there to dissipate. But that's not quite relevant at this point.

In theory and in an ideal situation (no system losses), it will take the heater exactly 4x as long to get to the thermostat setting using 120V compared to giving it the full 240V. But nothing is ever ideal in real life.


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