Sizing a generator for the house......

[teddy12b]

*Disclaimer, I'm not an electrical wizard at all. I know/understand very little about electrical work.

I have a friend from church who stopped by the house and we've been talking generators for the house. My garage is wired and ready for a portable gas powered generator to plug right in. The goals are simple, 1.) be able to run the ac/heat and also the have the sump pump powered. The furnace is natural gas so all that's needed is the control panel to be powered. Anything else getting powered above that is just icing on the cake. At this point, all I have to do is pick out a generator brand and the size of it.

I've been focusing my search on Generac's. I know there's other good ones, but that name brand seems to be the consistent one that comes up when talking about quality generators. I'm open to suggestions, but just wanted to let you know where my head was at the moment. As far as the amount of running watts goes my friend says I won't need all that much and an 8,000 watt unit would more than take care of what I'm looking to do. That size unit by Generac also comes with an electric start feature that'd be handy for the wife if I'm not around.

Any of you electrically smart preppers out there got any suggestions?

 

[Gluemanz28]

Hey Teddy

I'm going to be upgrading from a portable gasoline unit to a installed Natural Gas Generac unit. I want it to be automatic for my wife and to also be able to take care of itself if we are not home.

It is a little more money in the long run but my wife will have to become Amish if something ever happens to me. She just isn't good at stuff that requires technical abilities.

Then there is the whole gasoline thing of her having to put gas in it every eight hours if the power is off for an extended period.

 

[nra4ever]

I'd agree NG auto is the way to go if you can swing it.

 

[Tactically Fat]

OP:

Is your home ALL electric? If so, then 8k running watts may not be enough. If you're willing to do without heat/AC/hot water.... Then MAYBE.

If you have gas heat, gas hot water, gas stove/oven, etc... Then you MAY be able to squeak by on 8k.

My brother-in-law has a 17k NG Generac unit on his all-electric house. And it's a BIG house. The genset is plenty big enough.

ETA: Just looked yesterday. It's a 20kw Generac. He wants EVERYTHING to run "normally" when they lose power.

Other in-laws have the 17kw one - but have a few larger appliances also burning NG.

 

[nra4ever]

I have the 17k as well and it's plenty big enough. I'm sure there is a calculator online to figure out your needs. There is also a forum for questions.

 

[eldirector]

I've been looking at something like this for a while. We are on NG here, so have that option. Also on a well, so have that extra load to consider.

I can get by on an 8K if I manage what circuits are on at any given time. A 12K would run pretty much everything. A 15K does so without being loaded down so often.

My issue is managing the fridge/freezer, well pump, sump pump, furnace blower, and MAYBE the AC. These are all motors with a pretty high startup draw (decent running draw as well). I can either turn stuff on/off manually (turn off the well pump to run the AC, for instance), or step up to something that can do it all.

When I was young, we had a little 5K for emergencies. Would run the furnace blower (oil furnace), freezer/fridge, sump pump, and a few lights. We would also need to reset its breaker pretty often, when two things would kick on at the same time (sump pump had a HUGE initial draw).

Generac Power Systems | Portable Power | Generac Power Systems
Generac Home Backup Generator Sizing Calculator | Generac Power Systems

 

[lizerdking]

Similar situation, for $1k, 8kw does us fairly well. The whole house would have been five times that, and we don't use it but a couple few times a year. We're rural, and lose it for a few days sometimes.

Well Pump, Sump Pump, Electric Hot Water,Gas Furnace Blower, 3 fridges and a Deep freeze + Lights/TV/Computers.

The only caveat is if someone is going to take shower we turn off the hot water heater breaker for the duration of the shower, that keeps things from bogging down running the well pump full bore along with the hot water heater. When done, flip that hot water heater breaker back on and let it warm back up. It's not as great as running off utility power, but hey, it's supposed to be temporary right?


AC would be another matter, that's another 4kw at least...I suppose if you were desperate and it was a scorcher out, you could flip some breakers around and run just lights/AC to cool the house down, then flip things back...

 

[lovemywoods]

teddy12b:

Two other considerations as you design your system:

1. How are you going to separate the circuits that the generator is powering from the electrical grid? Just throwing the main disconnect is risky. You run the risk of powering to the pole and potentially injuring a power company worker who expects the line to be dead.

2. How will you know when the power from the electric company has been restored? There are some pilot light/buzzer devices to alert you.

 

[Leadeye]

I've got a Generac 14.4KW setup auto disconnect and it runs certain circuits in the house. Fuel is from 1000 gallon LP. Power goes out, some things come right back on.

 

[lizerdking]

teddy12b:

Two other considerations as you design your system:

1. How are you going to separate the circuits that the generator is powering from the electrical grid? Just throwing the main disconnect is risky. You run the risk of powering to the pole and potentially injuring a power company worker who expects the line to be dead.

2. How will you know when the power from the electric company has been restored? There are some pilot light/buzzer devices to alert you.

Everyone is soooo nervous about people throwing their main's. Truly, if you switch off your main, then start your generator, then do it in reverse when utility power is restored, you have NOTHING to worry about.


Not sure if it helps, but if you have a digital readout electric meter (most have been upgraded) it's off when the power is out, and on when it comes back...

 

[actaeon277]

Besides the size of the genny, there is the control panel to take into consideration.

Get a decent genny, you may get a decent control panel.
Some can sense when they are being overloaded, and drop circuits by priority.

A/C is going to be a big load.
sump pumps can be, but are intermittent loads.

 

[Gluemanz28]

Everyone is soooo nervous about people throwing their main's. Truly, if you switch off your main, then start your generator, then do it in reverse when utility power is restored, you have NOTHING to worry about.


Not sure if it helps, but if you have a digital readout electric meter (most have been upgraded) it's off when the power is out, and on when it comes back...

The OP said he wanted to have it so that his wife could also use it. I don't want my wife to have to deal with the issues of being prosecuted if she accidentally killed someone.

 

[Dave Doehrman]

Any of you electrically smart preppers out there got any suggestions?

Teddy12b, I think we've shot together up at Youngs a couple years back.

I built my new retirement home here in Ft Wayne and had the 20KW Generac unit installed when the house was wired. The 20KW is only $1,000 more than the 17KW unit and I have more than a "whole house" system. I can feed my workshop and a couple of the neighbors as well. In addition, if the natural gas system is disrupted, I can flip a lever, hook up to propane tanks and run off that.

If you want to take a look and see the unit operate, PM me and we can set something up.

 

[GodFearinGunTotin]

*Disclaimer, I'm not an electrical wizard at all. I know/understand very little about electrical work.

I have a friend from church who stopped by the house and we've been talking generators for the house. My garage is wired and ready for a portable gas powered generator to plug right in. The goals are simple, 1.) be able to run the ac/heat and also the have the sump pump powered. The furnace is natural gas so all that's needed is the control panel to be powered. Anything else getting powered above that is just icing on the cake. At this point, all I have to do is pick out a generator brand and the size of it.

I've been focusing my search on Generac's. I know there's other good ones, but that name brand seems to be the consistent one that comes up when talking about quality generators. I'm open to suggestions, but just wanted to let you know where my head was at the moment. As far as the amount of running watts goes my friend says I won't need all that much and an 8,000 watt unit would more than take care of what I'm looking to do. That size unit by Generac also comes with an electric start feature that'd be handy for the wife if I'm not around.

Any of you electrically smart preppers out there got any suggestions?

Here's a method to determine how much "running" kilowatts your equipment will need to run. Turn off everything you know you'll turn off in the event of an outage (where you're going to run your generator). Turn on everything you know you'll want to be able to run simultaneously. That means turn your heat on where the fan's running, etc.; turn on your sump pump (if you can without burning it up with no water in the sump); make sure your frig is calling for cooling, etc.

Then, go out to your electric meter and do the following. The number you get will be the minimum kilowatts. You will want to bump up the KW rating by 20-25% to allow for motor starting and unexpected loads.

Head outside with your stop watch and find the meter. If you have an analog meter you’ll see a metal disk spinning behind the glass front. If you have a digital meter there will not be a disk; you will instead see some sort of digital indictor blinking. That blink is known as a “beat”. Look at the front of the meter for a number notated with “kH”. The two most common numbers are 1.0kH and 7.2kH—if yours is different that’s fine. Write this number down.

If you have an analog meter, you need to watch the metal disk. The disk should have some sort of mark on it to indicate a start point in the circle. Wait for that mark to pass the indicator arrow on the meter face. When it hits that arrow, start the stop watch and wait for it to rotate all the way around. When the disk mark returns to the arrow, indicating one full revolution, stop the stop watch. This is the formula for calculating the power draw based on the spinning of the analog disk:

(3600 / number of seconds) * kH # = watts consumed

Let’s say that we watched our meter and the disk took 15 seconds to spin one full revolution. Additionally, the kH notation on the meter face was 7.2. We plug the numbers in like so:

(3600 / 15) * 7.2 = 1728 watts

That’s the real world consumption of that device at that very moment. By taking that number and plugging back into the formula from earlier in this guide (the conversion of watts to killowatt hours) we can calculate that the device we were measuring will consume 1.728 kilowatts per hour if it maintains that level of power draw and will cost us 21.6 cents per hour to operate.

To calculate the energy use on a digital meter, we use a similar counting technique and formula. If you have a digtal meter, go outside with the stop watch and get ready to count. Start your stop watch and begin counting the number of beats. Continue this for as long as you can stand it. Plug the numbers into this formula:

(3600 * beats) / seconds * kH # = watts consumed

The longer you can stand to count the more accurate the reading will be.

While measuring at the meter is more accurate than reading the label and running a guesstimate calculation, it still only shows us the power consumption at that exact second in time (albeit more accurately if you have a digital meter and a high tolerance for standing at the side of your house counting to yourself). Better than practically guessing but still quite prone to error as its difficult to isolate a single item in the house and impractical to measure it multiple times to get an average rate of consumption.

Obviously, this method is only accurate if your frig compressor and fan is running, same for your heat and/or AC, etc. The beauty is, you don't need an electric meter and you know what your requirements will be.

 

[actaeon277]

Generac Home Backup Generator Sizing Calculator | Generac Power Systems

 

[actaeon277]

I use a 4400 watt (peak) genny with a manual switch circuit breaker panel.

the panel let's me choose between line power, and generator. I can not use both, only one or the other.
Then I have loads below that.
Such as some outlets where my fridge and freezers are.
Heater control (nat gas heat, elec control)
and sewage grinder pump (my neighborhood has a weird system with a "grinder pump" on each house.

The grinder pump is a big 220 load. So I leave that off unless I have to. Then I turn off other loads to turn that one on.

AC is not connected. But if I get a whole house genny, that may change. After working multiple double shifts in a steel mill, and coming home to a hot house, I've determined it may not be as "optional" as I thought.

 

[indyjohn]

I have this:

3 hrs run time on a grill tank. 7800 running / 13000 peak watts.

And this:
Reliance Controls ProTran 310CRK Indoor Manual Transfer Switch Kit (30A)
Removes the worry of powering the pole.

A/C is not in the picture, but could be as stated earlier if everything else is turned off.

 

[Leadeye]

I have a spare 110 window AC, in the event of long term power out, we move into the basement.

 

[bulletsmith]

Go Big.

The calculations that have been put forth are a good starting point, but I must disagree with the amount needed to bump the size up for motor starting. The typical motor can draw between 6 and 8 times normal operating current for a brief moment when it starts. Logically you have to assume that all of the motors could start at once and provide for that. I've gone down this road a few different times and am of the opinion that 14kw is the smallest size I would recommend for just about any home with a sump pump and a well pump.

What is often missed is an understanding of what happens to devices that are fed with an AC voltage that has a frequency that is less than the standard 60 Hz. When your sump pump starts and you hear the generator change tone, sound different, you are hearing the governor sense that the speed of the generator has dropped to a point where it has to feed more fuel to the engine and increase the speed back to the target speed. The generator speeds up, assuming you have enough HP to meet the load, then the load drops and the generator is running too fast, fuel is then reduced. This ends looking like a dampened wave over time that shows how frequency reacts to the spike in load.

All of this means that your electronics in your home are being beat up with power at a frequency that is fluctuating well beyond the designed point. The end result is shortened or ended life of some of your electronics.

The bigger the generator, the more horsepower, and therefore the greater the ability to overcome those spikes in load with limited fluctuation in output voltage and frequency.

Sorry for the length, hope this provides some useful input to your decision process.

 

[indyjohn]

Go Big.

The calculations that have been put forth are a good starting point, but I must disagree with the amount needed to bump the size up for motor starting. The typical motor can draw between 6 and 8 times normal operating current for a brief moment when it starts. Logically you have to assume that all of the motors could start at once and provide for that. I've gone down this road a few different times and am of the opinion that 14kw is the smallest size I would recommend for just about any home with a sump pump and a well pump.

What is often missed is an understanding of what happens to devices that are fed with an AC voltage that has a frequency that is less than the standard 60 Hz. When your sump pump starts and you hear the generator change tone, sound different, you are hearing the governor sense that the speed of the generator has dropped to a point where it has to feed more fuel to the engine and increase the speed back to the target speed. The generator speeds up, assuming you have enough HP to meet the load, then the load drops and the generator is running too fast, fuel is then reduced. This ends looking like a dampened wave over time that shows how frequency reacts to the spike in load.

All of this means that your electronics in your home are being beat up with power at a frequency that is fluctuating well beyond the designed point. The end result is shortened or ended life of some of your electronics.

The bigger the generator, the more horsepower, and therefore the greater the ability to overcome those spikes in load with limited fluctuation in output voltage and frequency.

Sorry for the length, hope this provides some useful input to your decision process.

Quote this for truth. I've looked at startup load a lot, it changes your need a lot because you cannot reasonably be expected to monitor and adjust device demands all the time.