Simple generator sizing/wattage question


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Old 04-28-21, 07:53 AM
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Simple generator sizing/wattage question

Hello,

Simple question, but want to be sure I have it right, as I am trying to size for an external electrical backup generator. Live in a typical Colonial.

The electrical service that comes in is the typical 220 V line, and then goes to the box Main Breaker which is
labeled 200 (I presume amps).

So, what is the overall wattage "capability" that exists now for the house ?

Just 220 x 200, or is it 110 x 200 ?

Thanks,
Bob
 
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Old 04-28-21, 08:57 AM
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Amps (200) x voltage (240) equals wattage (48'000).

If you are getting a automatic standby generator you may not have the choice to pick what size you want or need. I'm going through it now and my local Building Inspections Dept. determines the size of generator based on the size of my home and how many appliances are electric and also includes extra loads like a sauna and hot tub. So, even though I routinely run on my small 2kw generator and only used the 12kw when we need to run the big items the smallest generator they would approve is 22kw. Even that was a battle. They wanted me to install load shedding controls for that size generator or to go to an even larger one.
 
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Old 04-28-21, 09:17 AM
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Barring nosy regulations like Pilot Dane ran into, hardly anyone needs a generator that can provide 200 amps, nor would most of us be able to afford one. You only need to power the essential things, which vary depending upon how comfortable you want to be when the power is off, and what types of equipment you have.

For example, I can get by with a small generator, just enough to keep a fridge cold, run a microwave, and keep a couple of lights burning. Gas supplies heat and hot water. Someone else may have an all-electric home with A/C, electric water heater, well pump, etc. Each situation is different.

Almost no one is rich enough and foolish enough to install a generator that can completely replace grid power. It just isn't necessary, nor is burning all that fuel a good thing for the pocketbook or the environment.


 
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Old 04-28-21, 06:35 PM
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Figure out the items/circuits you must keep running and add up the wattage. Also, give yourself a buffer for surge start-ups for things like A/C units. That is what size generator you should be looking at.
 
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Old 05-31-21, 11:55 AM
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If the generator system is supposed to be automatic then it has to accommodate any total load that might be in effect when the utility power fails (be big enough) and/or repudiate some of the load based on some kind of priority (have load shedding). Thus the building department's asking all of those questions and stating all of those requirements.

If the changeover is manual then you can select a generator that will meet your needs and affordability.. Thirty amps continuous with an added margin for motor startups (at 240 volts) is usually enough for a home that has gas or oil heat and minimal air conditioning. If you can always come home within a few hours of the power failure, or after work, to turn on the generator, then you will not lose the freezerful of food.

During periods of low usage, a smaller generator (that can handle the load) will consume less fuel than a larger generator from which the same total watts are being drawn.
 
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Old 05-31-21, 02:20 PM
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Yes, that is a very important distinction. When the power goes out I'm doing everything manually. All the circuits get shut off, we go through the house turning off anything with think was running. Then I bring on the little 2k generator and turn on the circuits I want. If we move from watching TV in the basement to up in the living room, I'm flipping circuits to only energize what we are using. Then I'd get out the tractor and hook up the 12kw PTO powered generator whenever we would run high demand devices like the well pump and air conditioning.

Years ago I truly realized and saw the extent of vampire power usage and just stuff that's plugged in and forgotten about. I would turn on circuits we weren't using and should have almost no load but the generator would kick up to high rev and groan. We have so much "stuff" consuming power. Bathroom, no it doesn't use any power except for my cell phone charger. Oops, forgot that the toilet bidet seat uses over 800 watts when it's heating water.

Now with an automatic generator. Everything that was on will likely be on when the generator comes online. Making it worse is everything is powering up simultaneously at a time when the generator's engine is cold. A tough application.
 
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Old 06-02-21, 02:05 PM
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The electrical service that comes in is the typical 220 V line, and then goes to the box Main Breaker which is
labeled 200 (I presume amps).

So, what is the overall wattage "capability" that exists now for the house ?

Just 220 x 200, or is it 110 x 200 ?
Oh, if it were just that simple, it would be easy to calculate, but extremely costly to install. You have to keep in mind that the majority of utilities only size their equipment and lines to provide the power necessary to meet your calculated load and NOT to meet the size of your service. Just because you have a 200 amp service doesn't mean you can use 200 amps.
 
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Old 06-03-21, 07:15 AM
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There are rules for load calculations. Load shedding devices can allow a smaller generator. Basically a whole house generator needs to be rated to run all connected devices and circuits, but with multiple circuits and devices, allowances can be made in the calculation for the likely percentage that might be running. Based on your question, you need to consult a professional, an electrician or a generator dealer.

I have a whole house 10kw generator serving a 2000 sq ft house with 200 amp service with 4 load shedding circuits for major electric loads like an oven and a dryer so that I can selectively use them. Total expense $6000 attained by careful negotiation between an electrician and the town permitting department.

Most homes with 200 amp service can use an air cooled 22kw or less generator, the least expensive type, with careful planning.
 
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Old 06-04-21, 11:00 AM
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When determining load shedding circuits, don't forget to add in the A-C unit which can come on occasionally when the power may be out while you are away from home.
 
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Old 06-04-21, 01:06 PM
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Load shedding should be considered as turning off appliances unless you are there to insure that the generator can handle the load. It is not a good idea to rely on unattended overload. Load shedding is primarily useful to legally install a smaller generator and to accept responsibility for monitoring loads.

If you have large A/C loads, and plan to leave them running unattended, the generator should be sized to handle them.
 
 

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