Standby Generator

[4estgump]

I need all the input i can get. I am thinking about installing a standby generator for my home. I live in a 4600 sg ft home. 2300 up and 2300 basement and garage. 15 ft x 30 ft wood shop down. My yearly average use is 2100 KWH a month. What size generator do you think i need to buy?
Thank You 4est

[DUD]

Are You intending to run the whole house? Bill

[BobSch]

I'm with Bill. Determine how much stuff you want to protect with the generator. Things like refrigerator, furnace (even if the furnace isn't electric, there's still the fan, circulating pump, thermostat, etc.), water heater (again, if electric), A/C, TV, radios, lights, etc.

Most permanent generator installs, include a transfer switch that will take care of a given number of breakers. That may be the fastest way to identify the capacity you need — figure out which breakers handle the stuff you want the generator to run and add up the breaker sizes.

[crokett]

Do you want automatic or manual switchover? Do you want whole house? If so, what kind of heat do you have? Gas or electric (heat pump)? If electric and you want heat you need one of the big 10Kwatt jobs. A portable one won't power a heat pump or A/C. If your heat is gas, you can use a portable to have heat but in summer no A/C.

I have a small portable 5Kwatt and installed a manual transfer switch a few months ago. I will power outlets/lights downstairs, water pump and fridge and some lights upstairs. I have heat pump so my heat will be a woodstove if needed. In summer my AC will be to move to basement where it is cooler.

[cabinetman]

Bob is right. You might be surprised how little you can get away with.
.

[LCHIEN]

unfortunately average use (2100 KWH/Mo) is of limited use in determining what your peak use (as must be specified for the genset) is.

Suppose you take your figure and divide by the number of hours in a month - ~720 then you will get an average draw of ~3 KW. But that would get you a severly undersized genset.

But, if you live in a heavily air conditioned area then your average usage in the summer months might be 3 times what it is in the winter months. So instead of looking at the averge over a year, look at your records for the highest use month and use that figure plus about 10 or 15% extra for safety. Lets say your heavy use month was July and you used 3500 KWh. So use 4000KWh as your summertime load.

A/C systems should only run about 1/3-1/2 the time (compressor duty cycle) under the heaviest summer load. So take the summer usage of 4000KWh and divide by 720 hours and you get about 5.5 KW. But, since the A/C is the biggest load (it amounts to about 60% of the bills here in Houston) then consider it 100% it draws 11 KW when running and 0 when off so to get an average 5.5KW then the peaks will be near 11 KW. YOu can refine it If you live in a cooler clime than Houston and your AC consumes a lower percent of the power, then maybe 8-8.5 KW will do.

This is the most extravagant way to size power backup - you assume the worst load and assume you will have everything on that you do when you have utility power. But its what you have to do if you have automatic switchover.

The cheap way to do it is to have manual switchover, and when you need to run the generator you pwoer off all non-essential items. Then you can get by with a smaller genset.

[Uncle Cracker]

And no matter what rating you go with, make sure the automatic (or manual) switchover completely disconnects your house from the normal power service grid. Power company employees have been killed by power backfeeding out from peoples' generators.

[master53yoda]

you can not size generators based on average load..................

the generator MUST be able to carry 125% of the connected load plus 150% of the largest motor load full amp draw. if the full amp draw on the Heat pump or AC is 10KW the generator must be 15 KW to start it. I have seen 12 KW generators not be able to start a 3 HP motor 1 phase motor IE a 3.5 Ton AC or heat pump.

A generator is not anything you want to miss size because to small and it either won't start the loads or you burn up the generator. I have seen many miss sized generators. The last full power conversion i saw on a 6,000 sq foot house was 30KW with a 300 cubic inch 6 cylinder ford engine. that might give you some idea of how big it may need to be.

A standby generator in a office building typically will only run the emergency lights, the elevator and any fire equipment that might be on sit.


the last full generator plant i worked on that served a hospital had 5ea 2 megawatt 16 cylinder CAT diesels.


Please remember that a panel conversion MUST remove the Utility company power when the generator is in operation. feedback from the generator through the power company transformers can put 12000 volts on the power company lines. Yes from you little generator that is rated at 5kw and 240 volt. A friend of mine was killed in an ice storm in the mid 90s due to an incorrectly connected generator.

[4estgump]

I have Geo-Thermal heat and a/c. My main problem is in the winter and storms in the summer. We live in the country and the electric goes out on a average of 5 times a year.
The Geo is 4 tons. I would like to run the whole house at these times. The longest outage was 7 days a few years back, from a ice storm. The shortest time has been about 30 mins. I do know about the switch box to stop electric flowing back into the service. Also i would like it to be automatic start. I knew i would get great knowledge
from ya all Kentucky.
4est

[master53yoda]

With a 4 ton unit you would need at least 20 kw. What the generator supplier will need will be the full load amp rating on the geo unit and if you can have someone connect a peak load tracking meter on the main panels to get the peak amp draw over a 7 day period, with those two pieces of information any one that is qualified could properly size your generator.

What kind of fuel did you intend to run it on. Diesel would probably be your best fuel but you may want to look at propane as a secondary option. The problem with gasoline is that it has storage problems that would need to be addressed. I am assuming that natural gas is not available. Is your home on a 200 amp service or a 400 amp. 400 amp creates another set of problems but it also may offer a solution if you chose to separate the loads into those that you intend to carry with the generator and those that you do not intend to carry. You could get a 200 amp auto switch that would drop out the power company from one panel and switch that panel over to the generator. That is a readily available component.

Take a look on the Net and see if there is a Cummins/ONAN dealer local. They would be able to provide you with the information as well as the support in connecting the system up.

Just a quick calc take the KW of the generator and divide by 240 volts and that will give you the amps that the generator can supply. So a 25 kw generator can supply 100 amps. and can deliver approximately 115 at start up.

As with all AC loads you can multiply amps x volts x .90 and it will give you approximate watts that the generator will use when the unit is operating at full load.

Also if you use a generator be sure and put your computer type components on an uninterpretable power supply to compensate for the variation in frequency that will come when your GEO unit starts. Start up of large motors will pull the speed down on the generator until the motor starts and could change the frequency from 60HZ to as low as 55HZ. This should not cause a problem with anything but your computer type equipment.

My background prior to retiring was a mechanical and electrical systems test engineer mostly on large commercial projects but I consulted on some smaller systems as well.

[LCHIEN]

I assume you're going to power from nat gas or something plumbed in that remains pressured (e.g. available) even when power is off. Gasoline takes too much maintenance (stabilizer, anti-varnish agents) and a weeks worth of supply is a lot of gallons that can go bad after time. Even propane requires large, unsightly tanks. Nat gas is the best (No storage and pay for upon use).

[master53yoda]

Cummins Crosspoint LLC - Louisville
9820 Bluegrass Parkway
Louisville, KENTUCKY 40299
Phone: (502) 491-4263

Web site: http://www.crosspoint.cummins.com

this is the closest Cummins dealer that I found I'm not connected to Cummins in any way but they have always been good to work with.

[4estgump]

I have been coming here from around 2003. I signed up in 2006. I come here 4-5 times a week. I don't post much, but have learned so much here. I want to thank the whole team for all your comments. They have been very helpfull. Where we live we do not have natural gas. So propane or diesel is the only choice. If anyone has more to add please do so.
Thank You All 4est