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PowerHouseMike

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Posts: 4
Reply with quote  #1 
Hey Everyone,

New to the forum. I was hoping you all might be able to help me understand a little more about off grid system sizing. I have done a fair amount of reading on the matter but sometimes it is just more helpful to have it explained by a person.

The project right now that I am looking at is for someone who runs 17,000 Watts for 18 hours a day. I will be using 24 Fullriver 24-DC1150-2 Volt AGM Battery, a Sunny Boy 7000TL-US-22 Inverter, and 4 Sunny Island 6048 Battery Inverters. I need to determine how many 320-watt solar panels I would need to allow the system to run for 1 day without energy. Ive done a bunch of the math but keep getting caught up and would love it if someone could break it down in a novice way. I am very interested in learning how to correctly size and design and off-grid solar system.

Thank you so much for your time,
Mike

stmbtwle

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Reply with quote  #2 
Nothing runs "without energy". Do you mean "without batteries" or "without solar"? I'm guessing without batteries. If you want battery capacity, Rick may be able to help.

For solar, go here, and work the problem backwards.
https://pvwatts.nrel.gov/pvwatts.php

Enter a "Trial" system size (you can use the default of 4kw). Once you determine the resulting output for your area, compare that with the required output, and adjust your array size accordingly. It's not rocket science.

For reliability you'll want to use the WORST month in your calculations. If you use the average or summer, you won't have enough energy in the winter.

Also consider that if your batteries are low, you have to have more energy to charge them.

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Solar is like the wind. It may be free, but putting it to work isn't!
Willie, Tampa Bay
PowerHouseMike

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Reply with quote  #3 
I looking to get alot more technical then that website has the capability of. Thank you though.
jjackstone

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Reply with quote  #4 
Mike,
It would help to know what you are trying to accomplish. As I understand your post, you will be drawing 17kw of power for 18 hrs per day. You have 24 - 2 volt 1150Ah deep cycles that I am guessing you want to series up into one 48 volt battery pack with a storage capacity of about 55.2kwh.

Since you have batteries, you will need a charge controller to charge the batteries. 
You seem to be using a combination of inverters. I don't know if either of your listed types can also charge your battery pack. 
I can do some basic calculations for you and you can go from there.

17kw x 18 hrs = 306 kwh per day used
If we assume an average of 5 good sun hours per day then,
306kwh / 5 hrs = 61.2 kw/hr need to be produced by your panels.
61200 watts / 320w per panel =191 panels
So 191 panels just to meet your base needs.
This doesn't include charging your battery pack, inefficiencies in charging, wire losses, inverter losses, etc.

As stmbtsle said, you have to plan for your worst sun month of the year. That will depend on where you live and your normal local weather conditions. I've simply provided a base that you can go by to help with the calculations.
JJ

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PowerHouseMike

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Reply with quote  #5 
This is awesome JJ thank you!

Your are correct about the batteries, they would be aligned in series to 48 volt battery pack. In this scenario the Sunny Boy 7000TL-US-22 Inverter acts as a charge controller. My calculations came out to about as many panels which is not feasible. 4.4 hrs of sunlight would be the number to go by as it will not be used during the winter months.

Also how would the equation be effected if there was a gas powered generator back up so that the system does not have to account for more then 1 day of no sunlight, or possibly less.

I appreciate your help JJ!

Thanks,
- Mike
stmbtwle

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Reply with quote  #6 
I'm still trying to figure out how you're going to use a Sunny Boy as a charge controller. Sunny Boys are GRID TIED inverters with an AC output. As far as I know they're not designed to work with batteries.

Sunny Islands on the other hand are designed as battery-backup inverters and as far as I know they are NOT solar powered.

You're planning on hooking up 61kw of solar panels to a 7kw inverter? Sounds like a recipe for disaster to me. You'll need about 8 more inverters, and you still need to charge your batteries.

With a 17kw load your batteries are probably good for a bit over 2 hours. So you probably need 8 more strings of batteries to get those 18 hours of run time.

As far as tbe gas generator is concerned, that's a matter of a changeover switch, either automatic or manual. You select generator or inverter, but not both. I understand some have connected a grid-tie inverter to a generator, but it's not recommended as it can get "expensive".

I think it's time to go back to tbe drawing board.

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Solar is like the wind. It may be free, but putting it to work isn't!
Willie, Tampa Bay
PowerHouseMike

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Posts: 4
Reply with quote  #7 
Awesome, thank you for your input.

This system is theoretical as it would be for a client that would never have the capacity to facilitate a 150+ panel system. This was my first dive into trying to figure out how to design an off-grid system.

Is there a way to design an off grid system that would support a 17kw load that would now require an obsurd amount of panels?

I appreciate all the help, just trying to learn.

Thanks all,
-Mike
jjackstone

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Posts: 75
Reply with quote  #8 
Quote:
Is there a way to design an off grid system that would support a 17kw load that would now require an obsurd amount of panels? 


Well 17kw is about 23hp. So you could up your generator size to provide a larger overall percentage of power. Maybe a 10kw generator and 7 or 8kwrs worth of solar panels. You'd still need at least a hundred panels. Of course then you would need some type of combiner to use solar, batteries, and the generator simultaneously and that is beyond my scope of knowledge. Good luck with the project. 

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stmbtwle

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Reply with quote  #9 
It can be done, utilities do it all the time, with several power sources feeding into a common "grid". On a smaller scale it's called a mini-grid. The info and hardware is out there, but that's about all I know about it.

Here are some links that may be of use:
https://energypedia.info/wiki/Mini_Grids
https://www.usaid.gov/energy/mini-grids/technical-design/components
https://www.usaid.gov/energy/mini-grids/technical-design/key-steps
https://www.solar-electric.com/commercial
https://www.civicsolar.com/support/installer/questions/can-4kw-generator-be-used-supply-grid-tied-75kw-inverters-signal-voltage

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Solar is like the wind. It may be free, but putting it to work isn't!
Willie, Tampa Bay
Rick H Parker

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Reply with quote  #10 
Sunny Boys are GRID TIED inverters with an AC output. As far as I know they're not designed to work with batteries.

Correct ... no charge controller in the Sunny Boy.

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Rick H Parker
Kansas, USA
Electronics Engineering Technologist
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