# What size generator for my house?



## gsdive (Jul 3, 2021)

I'm looking to add a bit of real backup power to my house. Current setup is a 1800W generator that only runs the sump pump via extension cord when needed. OK for short-term outages, not too great mental health-wise for the last outage that lasted 7 hours overnight in a Wisconsin winter.

The setup I'm looking at would be an 8 or 10 circuit transfer switch (depending on cost and availability) with a portable generator connected through an inlet box mounted on the outside of the house.

Analyzed my needs by using a clamp-on meter for each circuit at my main panel. Needs include sump pump, furnace, kitchen refrigerator, separate chest freezer and a couple of other general use circuits. General use includes the living room which has TV and few lights (to keep up with weather during an outage not to mention boredom battle) - only one of the LR circuits is really connected to much of anything at any moment - and the receptacle next to the main panel which powers various electronics - modem, router, satellite tv/antenna amplifiers, etc. Results in the table below.










Keep in mind that I as I checked each circuit I made sure to turn on EVERYTHING powered by that circuit that I could. E.g., the LR-north circuit also has multiple light fixtures such as porch/exterior lighting, front hall lighting, etc. Similar for the LR-south circuit. Most of these other devices are seldom on so these numbers would be a very worst case scenario. Getting the freezer to start on command turned out to be more time-consuming than I anticipated, so I'm assuming a similar number as the refrigerator circuit.

Depending on circumstances some of these circuits won't even get turned on. In the summer with a short outage the furnace, fridge and freezer may not get turned on, for instance. The flexibility of using the transfer switch shows its advantages.

If my measurements are accurate I'm seeing 2000 running watts with a max starting wattage around 3500W. (Again, leaving most of the lights off on certain circuits should reduce actual draw to 1300W/3300W.) The question now is what size generator is best. I see a number of generators in the 4000/4400 starting watt (3200/3500 running) range. These would be fairly economical to purchase. Next step up is around 6500W (5500W running). Obviously more money. It seems that a 6500W unit will never get above idle in my case. Also, I know having too much excess power that will never get used means the generator won't be running in its optimal range which could be detrimental long term. 

The idea of installing a propane conversion kit is also appealing. This generator will be a very infrequent use and being able to use propane would help eliminate concerns with stale gasoline/gummy carbs/etc.

Any thoughts/comments appreciated.


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## Browse Deweb (Jan 16, 2021)

You don't have any 240V breakers or appliances like air conditioners, pool pumps, etc.? Always oversize, so the generator doesn't have to work as hard. What is your budget? If you go with a larger unit, you can power the whole panel...even if you're only drawing a few thousand watts total. No need for a transfer switch, just get an interlock and switch on what you want.

I have the Honda EU7000is with the US Carb tri-fuel kit added. It's not cheap, but it's perfect for our needs and it's very quiet. There are much cheaper brands out there. Get a good inverter unit and you should be fine for occasional outages.


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## drmerdp (Apr 10, 2018)

Your calculations look thorough and in all likely good you can get away with a ~4000 watt generator. A 120/240v model with a l14-30 outlet is needed to make the best use of your transfer switch. In my experience 5500w generators in the 389cc range are a Nice compromise between fuel economy and output. 



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As @Browse Deweb, mentioned. You don’t have any 240v appliances?


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## FlyFisher (Jun 30, 2018)

drmerdp said:


> In my experience 5500w generators in the 389cc range are a Nice compromise between fuel economy and output.


That would have to be a rotary generator, especially considering the split phase operation mentioned thus far in the discussion for assisting with the transfer switch circuitry.

However, I think the OP's numbers look sound and I like the theory of "powering up everything on each circuit", even though in practice that isn't likely.

With those numbers - take the ~3700w number as a "high value" - a ~4000w class generator would be a good place to start. Your 3700w or so fits right in with the running wattage, even though that number appears to be on the high end factoring in start up loads. So if your generator, say, is a 4500w starting load generator with a 3700w running value your 3700w peak/starting load is that far under your generators' rated 4500w starting output. Doing the math there that puts you at 82% on start up - not bad. If you take out your starting loads and go to running loads (I am using the 1987 rounding up to 2k) then you are down at 44%. Some will say to keep the wattage draw on a generator to between 50-75% as that will keep you from working the generator too hard and keep the load on the generator enough to "work it" so it isn't "idling" with your load. If you look at the 44% number you are below that, but on start up loads at 82% you are up over that 75%. I wouldn't worry about that. Again, that is with a 4500w starting load generator. So the numbers look pretty good there. 

On top of it you can get inverter generators in that 4500w class. So you will save a lot of fuel and get a quieter generator than a rotary there also. That sounds like a win-win scenario.


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## drmerdp (Apr 10, 2018)

This is a bit more favorable. 








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## gsdive (Jul 3, 2021)

Browse Deweb said:


> You don't have any 240V breakers or appliances like air conditioners, pool pumps, etc.? Always oversize, so the generator doesn't have to work as hard. What is your budget? If you go with a larger unit, you can power the whole panel...even if you're only drawing a few thousand watts total. No need for a transfer switch, just get an interlock and switch on what you want.


No 240V circuits that I need to have operating during an outage/emergency. (The only active 240V is the A/C. There are a couple of other 240V breakers for future electric range & dryer; current appliances are nat gas.) As noted, this is Wisconsin - not Texas or AZ. If it's summer as long as the fridge and freezer are cold and making ice we should be able to survive a few hot hours or days.  

Coming from a commercial construction management background I think more along the lines of "emergency" backup - as in "what is actually needed to operate". The circuits I've looked at are the ones that, realistically, are used 90% of the time normally. (If you take the alarm clocks in the bedrooms out of the equation that goes up to 99%.)

I did look into interlocks. While cheaper, from my perspective a bit more complicated in operation. Turn off main, go through the panel and turn off what you don't need and then turn on generator circuit. Then later, turn off generator circuit, turn on everything else, finally turn main back on. The transfer switch scenario has already made the "which circuits should be on" boiled down to a few very specific, obvious switches - start generator, flip a couple of switches; if not not sure flip them all. Plus, by having some circuits always connected to the utility when the power comes back something, e.g., an alarm clock, should come on and let me know it's time to stop the generator. She doesn't know it yet, but even my wife could easily deal with the transfer switch - and she usually hesitates to get a screwdriver out of my toolbox.


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## gsdive (Jul 3, 2021)

Thanks for the comments. Always good to know there's not some glaring error in the logic chain.


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## Airstreamer67 (Oct 3, 2020)

Yeah, I like my transfer switch too. When power fails, I simply pull the switch down and it disconnects the desired circuits from the utility grid and connects them to the generator. 

When the utility grid power is restored, a clock on my oven, which remains on the utility grid side, comes on and lights up, thus alerting me that the utility grid is hot again, so I then push the transfer switch back up to reconnect to the utility grid and turn off the generator. 

Other than an automatic transfer switch device, it's about as easy as it gets.


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## ajnuzzi (12 mo ago)

Typically you'd like your generator to ideally run at 50% capacity. So if your running watts draw is going to be 4000, then you'd want an 8000 running watt generator. This is one of those cases where bigger is better. You won't run into a problem if your generator is too big for your load (aside from fuel consumption) but if you don't go big enough you'll definitely regret it. I have a 2200 sq ft house with all gas appliances and a 9500 running watt generator is enough to run 90% of what I need.


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