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Thread: Generator Size

  1. #1
    Join Date
    Jul 2011
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    17

    Default Generator Size

    I want to purchase a generator to run 2 sump pumps in my house. I am choosing between two sizes:

    Smaller: 3500w continuous / 4000w starting

    Larger: 5500w continuous / 6875 starting

    If you look on most generator sizing guides, they say that a sump pump pulls about 800w when running, and about 2000w when starting. Even running both sump pumps at the same time (that would be 1600w), the 3500w generator shouldn't break a sweat. Starting one while the other is running (800w + 2000w = 2800w) should also be OK.

    But you never know when sump pumps are going to turn on, so my concern is what will happen if both sump pumps try to start at the exact same time? Both generators have overload protection, so would that stop the generator from breaking? Also, how long would a sump pull that starting wattage. If it pulls 2000w for a second or two, then it's possible that both sump pumps could overlap when they try to start. If it's only for a fraction of a second, then it would be less likely for them to start at the same time (since that overlap window would be smaller). Also, if it happened, I wonder if one sump pump is starting, and the other would try to pull the wattage that isn't available, will it just strain for a second and then start a second later?

    My concern isn't the cost difference between the two generators...it is the gas usage. In NJ, this past week, getting gas during the outage has been a nightmare. The 5500w generator listed above runs (at 50% load) for 10 hours on 7 gallons of gas. The 3500w generator runs (at 50% load) for 12 hours on 4 gallons. That means that the 3500w generator consumes gas at about half the rate, which is a big deal when gas availability is sparse.

    Since I really don't need to run anything other than these sump pumps, I'd rather go with the more gas efficient model if possible. Thanks for any comments that you have!
    Last edited by ed3120; 11-04-2012 at 03:33 PM.

  2. #2
    Join Date
    Aug 2011
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    37

    Default Re: Generator Size

    For the scenario you are describing I would suggest a generator that has automatic-idle. Nothing drawing power, the engine idles.

  3. #3
    Join Date
    Aug 2007
    Posts
    6,486

    Default Re: Generator Size

    As long as the generator has overload protection, then the smaller one should suffice. Most that will happen is the breaker will kick if the draw is more than it can handle.

    Start up draws on a motor are basically the time it takes the motor to get from 0-full speed, not usually more than a fraction of a second. Also keep in mind that the higher the pumps have to lift the water, the more power the initial start will require and the higher the draw will be during operation.
    I suffer from CDO ... Its like OCD, but in alphabetical order, LIKE IT SHOULD BE!!!

  4. #4
    Join Date
    Jan 2011
    Location
    Columbiana, Alabama
    Posts
    623

    Default Re: Generator Size

    Both good points so far. Also, if two motors are side by side with one running and the other tries to start the one running actually acts as a generator for the fraction of a second and helps get the other spinning. This is called "motor contribution" and must be factored into "available short circuit" calculations.

    The 3500/4000W gen. is the best choice IMO and yours is a good question.
    Good Luck from Columbiana, Alabama
    Maurice Turgeon, Hidden Content

  5. #5
    Join Date
    Jun 2007
    Location
    Fayette County, Ohio
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    5,561

    Default Re: Generator Size

    Generator guides are nice but the usually have a built in buffer. I would suggest you check the specs on the actual sump pumps.

    Jack
    Be sure you live your life, because you are a long time dead.-Scottish Proverb

  6. #6
    Join Date
    Aug 2007
    Posts
    612

    Default Re: Generator Size

    Also keep in mind that once you have the generator, you will most likely find something else that you want to power, like a 'fridge or freezer. If all else is equal between the two generators, I would go with the larger genset because of this. I don't think the difference in gas usage will be substantial between the two units.

    Something else to think about....
    Consider a tri-fuel generator: gas, propane or natural gas. It has a carb on it that can burn gas, LP or natural gas. Propane stores much easier than gas. Natural gas is better IF the supply lines to your street don't get shut off during an event.

    Gas generators are not easy to use occasionally. The ethanol laced gas that is now available makes this tricky. You must treat the stored gas you have for the generator and ideally rotate it. You will need to run your generator routinely, probably once per month with some electrical loading. Some might say this is not necessary, but I spent the last week trying to help three neighbors keep their genny's running after they let them sit for the last 1 to 1-1/2 years. Mine is older than all three of theirs and still runs very smoothly because I did the above. Theirs required carb rebuilds, gas tank draining and fuel filter changes.

    I will be upgrading to a tri-fuel after the hysteria about getting a genny subsides. We have a large propane tank for the rest of the house (occasional heat, HW, cooktop) so that makes it the most convenient fuel for us.

  7. #7
    Join Date
    Jul 2011
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    17

    Default Re: Generator Size

    Quote Originally Posted by The Semi-Retired Electric View Post
    Both good points so far. Also, if two motors are side by side with one running and the other tries to start the one running actually acts as a generator for the fraction of a second and helps get the other spinning. This is called "motor contribution" and must be factored into "available short circuit" calculations.
    ***...that's pretty interesting. I didn't know that. My sump pumps are at opposite ends of the house, so I guess it doesn't matter.

  8. #8
    Join Date
    Dec 2010
    Location
    Houston Texas
    Posts
    2,371

    Default Re: Generator Size

    1- Be sure to plug the pumps into different circuits on the generator as long as your new "friends" don't need power.

    2- The moment you fire up that little puppy, your new 'friends' will arrive with an extension cord. I'd get the larger generator and I live in a state where it's legal to carry a concealed weapon.

    3- Be sure to get one with variable / automatic idle control. You don't want it running at max all the time.

    4- Be careful where you stock the 5 gallon cans of gasoline. VERY careful. Its the fumes that ignite, not the liquid.

    5- When the storm is over, change the oil and run the generator dry of gasoline. Use all the remaining cans of gasoline in your car. Don't let the stored gas sit for too long as it goes 'bad' so to speak.

    6- chain down the generator with a VERY large chain and lock
    Last edited by HoustonRemodeler; 11-05-2012 at 08:14 PM.

  9. #9
    Join Date
    Jun 2007
    Location
    Fayette County, Ohio
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    5,561

    Default Re: Generator Size

    Oh , and never run the generator inside.

    Jack
    Be sure you live your life, because you are a long time dead.-Scottish Proverb

  10. #10
    Join Date
    Dec 2010
    Location
    Houston Texas
    Posts
    2,371

    Default Re: Generator Size

    Following hurricane Rita a family of 4 died when the generator was placed in the apartment closet to keep it from getting stolen.

    http://usatoday30.usatoday.com/news/...-26-rita_x.htm

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