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-   -   Load Calculation for Back up Generator (http://www.diychatroom.com/f18/load-calculation-back-up-generator-33874/)

GuitarPlayer 12-16-2008 03:18 PM

Load Calculation for Back up Generator
 
I need to know what size, back up generator, I will need for the following circuits:
1. 2- Refrigerators
2. 2- Heat Pumps---- 1-15Kw, 1-10KW
3. Water Heater
4. Pool Pump
5. 2- 20amp Lighting and Outlet Circuits
6. 1- Washer
7. 1- Dryer 240 volt 30 amp
8. 1- Microwave 12 amp 120volt
9. 2-Computer Circuits

I was considering a 20KW, LP fueled, with auto transfer switch. I wasn't sure how to derate everything since it will not be a continuous load.
Thanks for your help!

InPhase277 12-16-2008 03:33 PM

Are the loads coincident? What will likely run at the same time? If your two heaters run at the same time, that's already 25 kW.

Gigs 12-16-2008 04:08 PM

Do you really need to run your clothes dryer during a power outage?

You need to say what your intent is here. Is this an off-grid system?

Gary_602z 12-16-2008 04:14 PM

Quote:

Originally Posted by Gigs (Post 198949)
Do you really need to run your clothes dryer during a power outage?

Same for the pool pump! Is the water heater gas or electric?

Gary

Yoyizit 12-16-2008 04:56 PM

Some of these loads will pull surge current for a few dozen or hundred milliseconds; you need a steady-state plus a surge current spec on the gen.

cocobolo 12-16-2008 05:50 PM

Guitarplayer:
I hope you are not serious in trying to run all that off a generator. You'd need a massive great diesel powered beast for that.
And as for the comment inquiring if this is a solar system - not a chance!
We live off a solar system here with a backup generator.
When you say backup, we are all assuming you mean during a power outage.
How often and for how long does this occur?
In the real world when you have an outage, all you should be trying to run are items that are absolute necessities.
And I don't think that two heat pumps fall into that category.
How many people in the household. That makes a difference.

Billy_Bob 12-16-2008 06:11 PM

Following is an estimator which should help. Don't forget all the little stuff which can add up. Also generators produce "dirty" electricity which electronic devices do not like. So get a power line conditioner and/or ask the generator manufacturer what you can and can't power with the specific generator you purchase.

http://www.portagen.com/information.php?id=36

Power conditioner...
http://en.wikipedia.org/wiki/Power_conditioner

micromind 12-16-2008 06:13 PM

The loads total around 40 KW. Depending on how cold it gets, there is a possibility (though small) of the majority of them being on at the same time.

The 2 heat pumps are the biggest offenders, obviously, and so is the water heater. I assume it's electric since you're using propane to fuel the gen. (No natural gas).

You could get by with a smaller unit if you could lock out some of the load while the gen is running. This isn't as hard as it might seem.

ATSs usually have auxiliary contacts in them, it's just a matter of choosing the non-essential loads, and powering them through relays which are de-energized when the ATS in in the source 2 (generator) position. This is very common on commercial/industrial buildings.

Strip heaters on heat pumps are usually 5 KW each, so your 10 KW unit has 2, while the 15 has 3. If you locked out one strip on each unit, your load is now 30 KW. The water heater is another likely target. It's usually 4.5 KW. Now you're down to 25 KW. If you make it a point to not use the dryer while the gen is running, that's another 5.5 KW. You're now down to 20 KW.

A few things about generators; A propane one will produce about 80-90% of its rated power for the first few minutes while it warms up. If it's liquid cooled, the power for the block heater and battery charger for a little one like this will cost around $30-60 per month. When the gen is running, the refrigerator/washer/pool pump and especially the heat pump compressor starting will dim the lights MUCH more than normal. There is a slight possibility that a 20 KW gen won't start the heat pump compressors at all.

Transfer switches usually have an exercise feature that starts the gen once a week or so. They can be set to exercise with or without load. Most people set them to come on sometime in the middle of the night. I don't recommend using the automatic exercise at all. Very few transfer switches will tell you if the test has failed, thus you have no way of knowing if the unit will start during an actual power failure. My preferred method for a generator test is to personally make the ATS start the gen, and transfer the load. If this is difficult via the keypad, simply install a test switch in series with one of the source 1 (utility power) control voltage wires.

Rob

Gigs 12-16-2008 06:26 PM

Quote:

Originally Posted by cocobolo (Post 198997)
Guitarplayer:
I hope you are not serious in trying to run all that off a generator. You'd need a massive great diesel powered beast for that.
And as for the comment inquiring if this is a solar system - not a chance!

I said off-grid, not solar. Not everyone who is off-grid is solar.

jamiedolan 12-16-2008 07:37 PM

http://www.guardiangenerators.com/pu...0178480SBY.pdf

This is the level of generator required to support the load your speaking of. They are in the $10,000 to $15,000 range, plus install cost. Plus cold weather kits, load transfer kits, etc.

Jamie

TazinCR 12-16-2008 09:06 PM

Remember an emergency generator is for emergencies. It will set idol 99% of the time so that is money just setting idol.

Gigs 12-16-2008 09:23 PM

Quote:

Originally Posted by TazinCR (Post 199092)
Remember an emergency generator is for emergencies. It will set idol 99% of the time so that is money just setting idol.

If the price of copper keeps going up, a generator isn't a bad investment. Mine has gone up in value $300 since I bought it.

Billy_Bob 12-16-2008 09:27 PM

Quote:

Originally Posted by TazinCR (Post 199092)
Remember an emergency generator is for emergencies. It will set idol 99% of the time so that is money just setting idol.

Some "GuitarPlayers" make quite a bit of money! Perhaps the OP is in a similar situation and does not care what it costs?

One time I went to do some work on a customers home which had a 7 car garage, a guest house the size of a regular house which had its own separate pool and hot tub, then the main house had maids quarters and was just huge! Had a "head" gardener who had a PhD in horticulture, etc., etc.

Anyway they said they wanted me to do the work because they were going to their "summer home"!

GuitarPlayer 12-17-2008 08:07 AM

No 7 car garage here :laughing:. I am doing some research for an older couple that moved here, to tenn, from up north. They bought a huge log cabin up in the mountains, where power outages are quite frequent & an ice storm would take out power for weeks! Thanks for all your help :thumbup:

jerryh3 12-17-2008 08:29 AM

Quote:

Originally Posted by GuitarPlayer (Post 199225)
No 7 car garage here :laughing:. I am doing some research for an older couple that moved here, to tenn, from up north. They bought a huge log cabin up in the mountains, where power outages are quite frequent & an ice storm would take out power for weeks! Thanks for all your help :thumbup:

You'll also have to worry about the fuel source. A 20kw generator will use between 2-3 gallons of LPG an hour. If access to the cabin is limited, they will need a huge propane tank. They need to take this into account when pricing this out also. And, there's always diesel generators...


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