Generator Size

[Larry Frank]

I am looking at getting a portable generator. If the power is out, I would like it to run my well every now and then. The well is 4" with a 3/4 HP 230 Volt Red Jacket Submersible Pump that is down about 80 feet. I have no idea what the starting watts would be on this. Can anyone give me some idea how big a generator I would need?

[Scott T Smith]

Probably no less than 5KW.

I live on a farm and have several generators. One is an older Onan JB model propane 5500 KW genset and it is hard wired to an automatic transfer switch at the house. Via a sub-panel, it provides back up power to the well, the electric fence, the lights in the barn, the kitchen, the upstairs bathroom, our bedroom, the TV, the basement freezer and the furnace (propane). We also have a propane water heater, so we're in pretty good shape if we lose power in the winter. It won't pull the AC in the summer though; for that I have to bring my welder over from the barn (12KW generator built in).

Our well is 400' deep with a submersible pump. Be careful regarding some of the "capacities" listed for modern generators - the manufacturers often play games with the specs. A good way to double check is to assume that you will require 1.8 HP per KW of power, so a 10KW generator should have at least an 18HP engine.

There is a lot of good info available on the internet re generator sizing, and it would be best to add up all of the potential loads that you want to carry and run the calculations.

Propane is nice because you don't have to worry about the fuel gelling (diesel) or going bad (gasoline). If you have natural gas available, that's even better as then you don't have to worry about refilling the tank during an extended outage.

[Tom Veatch]

A 3/4 HP motor is going to consume less than 1 KVA on a continuous basis. Most of the sizing guides I've seen recommend a 3 to 1 surge capacity for motor startup. Based on that, a unit capable of handling a 2 to 3 KVA surge would meet your needs if all you're going to do is run the water pump.

Are you sure that all you want to do is run the water pump? If you're going to invest in a generator, you might want to keep the refrigerator and freezer running as well, not to mention ...

Do a google search for "standby generator sizing", you'll get a number of links to recommendations for generator size based on what you want to keep running during a power outage.

[Matt Meiser]

We've got a 5K continuous/6.5K peak unit that has served us well. I didn't do any shopping around when we bought it during a major ice storm--I bought what they had and was happy.

Ours is a gas B&S/Generac model from Home Depot. I keep it full of Stabil-treated gas and start it every 6 months and let it run with a load on it for a 1/2 hour or so and that seems to work well to keep it ready to go.

We run our well, lights, fridge, freezer and furnace on it. By shutting off lights, the fridge, and freezer, we were even able to do a load of laundry (propane dryer) during an extended outage. We don't cook with the microwave or gas stove. It also runs our camper's A/C--handy during the big blackout a few years back.

[Bob Borzelleri]

I hope this doesn't come across as a hijack, but I've wondered about setting up a generator to run specific appliances/lights, etc.

Perhaps one way is to hook up the generator to come into the main panel and be available for whatever you turn on up to the point where it gets overloaded. I guess the alternative is to bring in the generator line to a sub panel that goes out to just those items you want to bring power to.

In the latter case, it seems that the specific selected circuits would have to have been run with backup power in mind in the first place. What am I missing here?

[Tom Veatch]

I hope this doesn't come across as a hijack, but I've wondered about setting up a generator to run specific appliances/lights, etc.

Perhaps one way is to hook up the generator to come into the main panel and be available for whatever you turn on up to the point where it gets overloaded. I guess the alternative is to bring in the generator line to a sub panel that goes out to just those items you want to bring power to.

In the latter case, it seems that the specific selected circuits would have to have been run with backup power in mind in the first place. What am I missing here?

Typically there's a transfer switch installed between your main panel and a subpanel that contains the circuits you want to have available when running on backup power. That transfer switch is a two position or double throw configuration. In one position, it connects a feed from the main panel to the subpanel and disconnects the feed from the generator. In the other postion, it disconnects the main panel and connects the emergency power to the subpanel. The transfer switch prevents an interconnect between your generator and the utility grid.

You absolutely do not want a situation where you try to backfeed the utility grid. It doubtful your generator could handle the load, and, more importantly, you'll really P.O. the guys working on the lines.

[Bill Arnold]

This has nothing to do with the OP. You've gotten soem good info already. The OP reminded me of the Honda generator ad on TV that shows power going off, then a woman goes out to the patio to hand crank what appears to be a 2KW generator with an extension cord running from it and the entire house lights up again! :confused: :eek:

[Dave Johnson29]

Perhaps one way is to hook up the generator to come into the main panel and be available for whatever you turn on up to the point where it gets overloaded.

That is a recipe for certain disaster when the power comes back on. You need a transfer switch. With 200-Amps coming into the house your generator would be toast in seconds.

[Jay Jolliffe]

I have a Honda 6.5K that runs my whole house. The electrician set it up so when the power goes out pull a lever that's marked generator or power. I put it on generator flip the switch and the house is full electric. You do have to use your head as what you want to run. For me it's the well pump, refrig, a few lights & maybe heat. We do use a wood stove also for heat. So far the longest it's had to run is 3 days & worked great.

[Bob Borzelleri]

That is a recipe for certain disaster when the power comes back on. You need a transfer switch. With 200-Amps coming into the house your generator would be toast in seconds.

Dave...

I am aware of the need for a transfer switch. My question is specific to whether the generator needs to see predetermined circuits (as in just the minimum lights and appliances, etc.).

Maybe a better way of asking the question is this. If you have not had your house wired with the use of a generator for specific circuits, then what is the standard approach for figuring out where the generator power will go, particularly when you might have 6 desired destinations and they are all on different circuits.

...Bob

[Bob Borzelleri]

I have a Honda 6.5K that runs my whole house. The electrician set it up so when the power goes out pull a lever that's marked generator or power. I put it on generator flip the switch and the house is full electric. You do have to use your head as what you want to run. For me it's the well pump, refrig, a few lights & maybe heat. We do use a wood stove also for heat. So far the longest it's had to run is 3 days & worked great.

Thanks Jay...

That's what I was wondering about. Do you have a sense for what output you would need to run your whole house?

...Bob

[Stephen Reid]

There is a redneck way to hook this up.I'm not liable for the results!Do so at your own risk.Gear up a extension cord cord with male on both ends.Make absolutly sure you shut off your main service breaker so you don't try to power the whole grid or fry a lineman.Do not forget to do this!!!!!Shut off all the breakers that you dont need and then fire up your genset. Plug into the gen and then to an outside plug on the house.Let there be light.Before I get all the hatemail yes I had to do this years ago when we lost power for over a week after hurricane Juan and had to borrow a generator.Worked like a charm YMMV . I wouldn't buy anything less than 5000 running watts

[Myk Rian]

Maybe a better way of asking the question is this. If you have not had your house wired with the use of a generator for specific circuits, then what is the standard approach for figuring out where the generator power will go, particularly when you might have 6 desired destinations and they are all on different circuits.

...Bob
I have a 220 receptacle in the garage. A 220 extension cord with male plugs goes from there to the generator. I disconnect the main in the house before starting it up. It powers all the house circuits, but we keep the usage down when on gen. power. It's a 4400 w unit and does quite well.

[Tom Veatch]

Dave...

I am aware of the need for a transfer switch. My question is specific to whether the generator needs to see predetermined circuits (as in just the minimum lights and appliances, etc.).

Maybe a better way of asking the question is this. If you have not had your house wired with the use of a generator for specific circuits, then what is the standard approach for figuring out where the generator power will go, particularly when you might have 6 desired destinations and they are all on different circuits.

...Bob


Determine which circuits you want to receive emergency power.
Move those circuits to a subpanel that is fed from the transfer switch
Feed the transfer switch from the main panel/generator.

[Scott T Smith]

Dave...

I am aware of the need for a transfer switch. My question is specific to whether the generator needs to see predetermined circuits (as in just the minimum lights and appliances, etc.).

Maybe a better way of asking the question is this. If you have not had your house wired with the use of a generator for specific circuits, then what is the standard approach for figuring out where the generator power will go, particularly when you might have 6 desired destinations and they are all on different circuits.

...Bob


Bob, let me add a bit to what Tom Veatch recommended (which I concur with).

The safest way to use a backup generator on a home is via a transfer switch. However, the transfer switch has to be sized according to the total load on the panel that it supports - not according to the size of the generator.

If you do not want to split off your critical circuits onto a subpanel, then your transfer switch will have to be sized based upon your incoming power. For most applications, that's 200 amps, and in some instances 400A.

If you size the transfer switch based upon a subpanel, then you can drop down to something like 30 - 50 amps, depending upon the total load on that subpanel.

Needless to say, a 30A transfer switch costs a heck of a lot less than a 200A transfer switch - especially if it is an automated version.

So to answer your question, "no", the generator does not need to "see" predetermined circuits, but if it doesn't you will likely spend much more $ on hardware than you need to. There's tradeoff's here - if you're paying an electrician to do all of the work then you need to compare both the hardware as well as labor costs.

[Bob Borzelleri]

Wow! Now I really feel like a hijacker.:cool: Thanks to all for the input. When we built the house, I asked the electrician to give us a sub panel for a generator that would supply the entire house. The sub panel is hooked up to an input stub on the far side of the garage.

Now that we are finally getting serious about putting a generator, I started thinking about how the house is wired and spoke to a fellow who was involved with the original wiring. He thought that the sub was set up for the kitchen and a few lighting circuits. The original electrician is long gone so I guess it's time to have someone track down the circuits and tell me what I have.

Thanks again for the info and apologies to the OP.