DIY Home Improvement, Remodeling & Repair Forum > How to determine power requirements for a generator and other questions

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07-30-2010, 11:45 AM
ilyaz
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How to determine power requirements for a generator and other questions

So now that I understand that I can't place a gas-powered generator inside my house, I will need to figure out where I would put it on the outside (since there is relatively little available space).
- The 1st question is how powerful should the generator be. To determine the max Watt requirements, can I simply take every breaker in my breaker box, multiply its amperage (say 15 amps) by 110 Volts and get the power needed by all the electrical pieces that sit on that breaker? I would then sum up the numbers for all the breakers, or just those that control vital equipment (e.g. fridge + AC + some lights in the summer) to get the max wattage the generator should produce?
- In general, given the same power level of a generator, does one powered by natural gas take more/less space, generate more/less heat, is more/less noisy, more/less reliable, more/less expensive to operate than an equivalent one that uses diesel fuel?
- How close/far can the unit be to the house? How much empty space should there be around the unit?
- Any other aspects I should consider?

Thanks much!

07-30-2010, 07:20 PM
Blue Jay
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As far as size figure out what you need to keep running, Fridge, freezer. furnace (for winter time), sump pump, some lighting. Check each appliace you need to keep working, multiply the amp rating by 120 (not 110) to get wattage needed add at least 20% for a min. size unit. This would be a whole house unit as that would be a pretty big generator around 30KW+. They will make some noise but not bad. The only difference in size would be for a fuel tank for say a diesel or LP powered unit. Would need enough space around let the unit enough air for cooling.

07-30-2010, 11:24 PM
Nestor_Kelebay
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I'd approach this problem a bit differently.

I wouldn't go with natural gas or diesel. I'd go with a gasoline powered generator so that I could keep a supply of gasoline on hand, and if I didn't use it in an emergency for the generator, I could always use it in my car for non-emergency driving. Oil companies change their formulations from summer to winter for easier starting, and you don't want to be trying to start your generator in summer with winter fuel and vice versa. Going with gasoline will allow you to use old fuel in your car if you don't use it in an emergency.

I'm thinking that you might want to allow for 1500 watts for a fridge and another 1500 watts for a freezer. 400 watts for a circulating pump or blower on your house's heating system (and the control system on a typical house is typically powered by a 40 VA transformer, so maybe allow for another 40 watts for the heating controls). Another 500 watts for a sump pump and say 1000 watts for a microwave oven or hot plate to cook with OR watch TV news reports with OR plug your car's block heater (count on 100 watts per cylinder, and after 4 hours your car's engine will be as warm as it's gonna get; any longer and you're just wasting electricity) in during really cold weather OR wash clothes in your washing machine with.

If you switch over to compact fluorescent lighting now, then another 100 watts will be enough to allow for 8 lights burning simultaneously, and that's way more than anyone needs during an emergency.

That adds up to a total of 5000 watts. But, if it was me, I'd increase that by 50 percent to provide plenty of headroom for unanticipated loads (like a pair of electric water pumps to pump out the water that leaks through your sandbag dyke or to operate a few wet/dry vaccuum cleaners to remove the water from your basement). However, a 7500 watt generator will probably only provide 120 volt power, so you're not going to be able to use any of the 220 volt appliances in your house like a stove or electric dryer.

Here's a diesel 25 kilowatt generator that'll only cost you a mere \$21,000 before taxes:

Multiquip Ultra Silent diesel generator model DCA-25USI2C

And here's a gasoline powered 7,500 watt generator for \$700 that will fit in your garage:

Amico - Amico 7500 Watt Gasoline Generator w/ Wheel Kit & Electric Start - AH-7500E #AH7500E

Personally, I think the 7500 watt gasoline generator would be the more practical choice unless you're house doesn't have electrical service and you're going to be relying on this generator for all of your electrical power needs indefinitely.

PS: Can't keep a gasoline powered generator inside your house? You need to learn how to deal with the fire inspectors. I kept a gasoline powered snow blower in my boiler room for years, even when I had both a pilot light on my boiler and a pilot light on my water heater. I just showed the inspector that the plastic gas tank had been removed from the machine, drained, and is kept in another room. I only put the gas tank on when I need to use the snow blower. I expect no one would have any problem with your keeping a gasoline powered generator in your house if the empty gas tank was in your garage. All you have to do is splice a tee into the gas line from the tank to the engine's carburetor, and have a valve on the line coming off the tee. After using your generator, put the gas line the valve is on into a gas can and open the valve to drain all the gasoline out of the tank. Run the engine just to use up the gasoline in the carburetor so that you won't have a problem starting the engine with the old stale gasoline next time. An empty gas tank won't weigh more than 20 pounds, tops.

You should also find out what the requirements are regarding transfer switches. In the little bit of looking I did on the internet, all the transfer switches I saw stated that they were for generators of 7500 watts and above. Find out if you can get away without a transfer switch if your generator is less than 7500 watts. (I'm just guessing that without a transfer switch, you might just have to connect the output of the generator to the 50 amp receptacle for your stove for distribution of power throughout your house.) In that case, you'd have to have your generator outside (maybe in a ventilated enclosure), and have a cord rated at 50 amps connecting the generator to the receptacle from your stove (which connects to the main electrical panel of your house). That would allow power from the generator to be distributed to all of the electrical circuits in your house so that all of the lights and wall switches would work the same way as normal.

Last edited by Nestor_Kelebay; 08-01-2010 at 04:35 AM.

02-28-2015, 06:27 PM
Eddie_T
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I don't advise doing this, but I use a 3.5kw gas generator that I wheel out of the garage and using a cord with male 240v connectors on each end backfeed a 240v circuit. I switch off the main breaker and any circuits I don't want or need. Usually I keep water pump, fridge, freezer and a lighting circuit on. If I want to use the electric range I turn off fridge and freezer as they don't have to be on all the time.

Again, I do not recommend this approach but I am comfortable with it.

03-01-2015, 05:45 AM
beachguy005
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You need to take a real hard look at the fuel you'll be using and how you're going to be storing it. When you total the combined load of all the items you want to run, especially the ones at the same time, then size the generator, look at the fuel burn rate per hour. So you calculate a need for a 5kw genset and see that it may have an 8 hour run time burning 5 gals of gas at full load. Start adding up how many hours you may need to run it and the fuel you need adds up pretty quickly. Getting more fuel when you need a generator for power may not be too easy.
I'm in south FL and my decision was on a small 2ooo watt Honda portable. I'll use it for my fridg, tv and a few portable fans. It's small, light, quiet, doesn't use a lot of gas and easy to take with me in the event I have to leave my house. Great for non-emergency use also.

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03-01-2015, 09:06 AM
Big Red
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I've had a stand-by generator for 12 years which runs off of propane since I don't have natural gas way out here in the boonies. I sized it, not to handle absolutely EVERYTHING, but only what I would need during an outage. If you can't include everything without getting into too big of a generator that will cost you tall money, then only choose to dedicate what you need during an outage, as I did. I have dedicated outlets, the furnace, frig, freezer, stove, blower on the woodburner, sump pump, and septic aerator,etc on the dedicated circuit. I have a 12K Generac that I bought during the great blackout OF 2003 and I only paid \$2500 for it. I'm not sure what they cost today.

A stand-by generator is a great thing to have because you don't have to be home to start it up. I would locate a generator like this in an area that's away from the living and sleeping quarters--like next to an attached garage because they are kind of loud when running. And remember that they exercise themselves once a week too, so you don't want that noise in your living areas.

If you have a bad storm with trees down,etc., or a bad ice storm, you might not be able to get out to go get gasoline. So a generator that runs off of natural gas would keep your house up and running for WEEKS if needs be without you having to try to get out for more gasoline. I love my generator because living in an area where there aren't many people, and way off the road, one needs to be able to take care of oneself if a crippling emergency happens.