Generac generator sizing ?


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Old 06-17-24, 03:51 AM
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Generac generator sizing ?

Hello,
Great Forums and folks. If this is not the "best" forum to ask this of, please let me know.

Live in the typical 2 story Colonial in Mass.
Gets pretty cold in the winter, as you would expect.

Thinking seriously of going for one of those Generec natural gas electrical genertors.
The kind that would be permanently installed outsIde, and come on automatically in case
of a power failure.

Not sure what size to go for.
We have as expected 220 V coming in, but the paper stick-ons inside the
box are missing, so it's hard to know what we would require.

Would want, of course, enough capacity, with spare, for powering the furnace & its hot-water circulators,
all the house lights, our electric stove (probably a big drain), Dishwasher, and a TV or two.

Not sure what else.

Any opinions on what capacity we should be considering would be appreciated.
And, any thoughts on what to get with unit, installation hints, etc., etc. would be very appreciated.

e.g., are there different types of automatic transfer switches ?
Must it be placed inside the house, or are there good and safe outside installs for this unit also ?
Pros and cons ?

Thanks,
Bob
 
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Old 06-17-24, 04:31 AM
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Keep in mind that all appliances and fixtures will not be operating simultaneously. There are excellent websites and charts available that show power needed for each item. Beyond what you've mentioned, consider a refrigerator and freezer, hot water heater, microwave, washer or dryer. Find the combination that makes most sense to you. For example, you probably wouldn't use the washer, dryer and stove at the same time. When you're done, add 20% because you wouldn't want the generator to operate at capacity for long.
 
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Old 06-17-24, 05:29 AM
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The generator will be placed outside for noise and the exhaust. Typically around 5 foot from any building openings.

The automatic transfer switch will become the service where grounding and bonding takes place so the current panel will need some changes and a new 4 wire feed. The grounds and neutrals will need to be separated.

Check with your gas company to see if a gas main upgrade is needed.
 
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Old 06-17-24, 05:44 AM
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Size really depends on budget and need. I used a portable 5500 watt (that's on the small end). Kept basic lights, refrigerators, well and a few more things running. It could not handle HVAC. A 22KW standby generator will power the whole house and keep the HVAC running fine. When pricing, don't forget all the additional costs. The generator is just the first thing. Transfer switch, installation, electrical work, permits, possible gas main upgrade as Pcboss said. This is usually not a DIY project unless your really skilled. There are not that many options out there. Only a couple of companies offer the larger standby models.
 
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Old 06-17-24, 05:44 AM
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You will require a permit for installing a automatic standby generator. Your codes may dictate what size you are REQUIRED to get. Because the system is automatic it must be sized to meet the possible load placed on it. When I did mine a couple years ago the Inspections Dept. considered the size of my home, whether the heat was electric or gas, water heater by gas or electric as well as our kitchen stove and clothes dryer.
 
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Old 06-17-24, 06:09 AM
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My current renovation of a bedroom and bathroom for aging-in-place also upgraded my electric panel. As part of that I had installed a separate 60 amp "critical circuits" subpanel for heating, refrigeration, internet, sump pump, etc. It is powered by a 60 amp 2 pole breaker in the main panel for now. If I later add backup power (permanent or portable generator, battery, etc. TBD), a transfer switch will be added for that panel only.
 
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Old 06-17-24, 06:47 AM
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With absolutely no knowledge of requirements and no info here about your service, it might be wise to get a bid or two from dealers. Then make your decision about how much DIY you want to get involved with.

I did that, and months of research and monitoring my power usage led me to buy online to my specs and hire a contractor that I had worked with before to do the permitting and install. My research allowed me to pick the minimum size, and cost, to meet a whole house installation with load shedding for safety.

Sadly, the power monitoring service that I used to graph my power use every 10 seconds no longer is in business.
 
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Old 06-17-24, 02:03 PM
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In the permit process.... you will be given a load calculation sheet that needs to be filled out.
You'll need permits.... electrical, plumbing and building if you pour a pad.
You must notify the gas company in advance of the size of the generator and anticipated requirement,
They will also ask you what size your meter is. It'll be on the label. (225k, 250k, 275k, for example)

If you have a 100A service then more than likely you'll be getting a 12k-14k generator.
As mentioned previously.... since the transfer switch is automatic... the generator must be able to run all connected loads..... unless loads are disconnected during generator operation.
Generac and Kohler have free estimate services where they'll come out and give you an estimate.
Some offer extended financing and warranty extensions if they do the work.

Don't be pressured. Get several estimates.
 
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Old 06-17-24, 03:02 PM
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Never heard of the county/state deciding what size generator you're allow. Don't doubt it's true in your area PJ, but I don't think Maryland has the same restrictions. Why would anyone care? If I only need 20KWs to run my house and I have a 30KW generator, then I am just not using the extra power and there is a little less load on the generator. And if I undersize my generator, then I will need to manage the load. Not sure why the county cares as long as I have the proper electrical, gas and building permits and inspections. Also, not sure why we would care what size meter you have. The meter is never used in the off-grid mode, when the generator is running. If installed properly, the transfer switch isolates your house from the grid and the meter never sees it,
 
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Old 06-17-24, 03:44 PM
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They don't decide what generator size you use but you must show that when the generator transfers it can handle ALL connected loads. If you want to get a smaller generator you need to show what loads will be automatically bypassed during generator operation.
 
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Old 06-18-24, 05:21 AM
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Previously I had used a 12kw manual generator without trouble. But since I had to set it up I would be able to turn off the electric water heater, hot tub, AC and not dry laundry. But because an automatic generator may come on at any time, even when you are not at home, it must be sized to handle the possible load. With the addition of load shedding (my transfer switch turns off the AC for 2 minutes when the generator is going to start) I was able to use a smaller 22kw generator. Without automatically shedding or turning off devices I would have needed a larger generator.
 
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Old 06-20-24, 11:03 AM
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The Generac website has lots of good information. It might be worth skimming the installation instructions even if you're going to get it professionally installed so you have the basics.

How often do you experience outages?
If you're in an area that has reliable service, but might get an outage once a year due to a big winter storm, it might be worth just getting a portable propane generator and a few 5lb (bbq size) tanks. Connect it via a manual transfer switch. Size it to run the fridge, a few lights, and some heat. The goal being you are a bit more comfortable for the few days you're out of power.

On the other hand, if you're in the country and see a few days of outages throughout the year, then an automatic generator/transfer is well worth the investment. AC is going to be your biggest draw, as will heat if you have electric heat. If it's gas/propane/oil heat, or even heat pump, you'll be able to run a much smaller generator as the heat isn't being generated by the generator.
 
 

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