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battery requirement calculation

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Posted by angus.berry on January 28, 2009, 5:58 am
I wonder if someone could help me with a battery sizing system
calculation to help me get a better understanding.

This is an idea conditions calculation!

Lets say a 150w, 12VDC appliance needs to run 24/7.

I have PV cells that produce 500W x 8 hours a day (every day of the
year :-)

What would the AH rating of battery bank need to be to fall to an
acceptable level overnight without damaging the batteries and sustain
this 'ideal' system, running 24/7

Thanks for any help getting me started in understanding this!

Posted by drcsolar@gmail.com on January 28, 2009, 9:13 am
On Jan 28, 12:58am, angus.be...@gmail.com wrote:

Watts/Wolts=Amps thats how many Amps your drawing during useage.
12V=12.5Amps run that for one hour you get 12.5 Ah per hour, now
mulitple that by 24 you get 300 Ah during a 24 hour period.

300 Ah and you want your battery discharge to not exceed 50% SOC so
you would need about 600 Ah of capacity.

And my personal preference would be to go with Trojan but you can use
what ever ones you want being pricing may be an issue.

Posted by Russ in San Diego on January 28, 2009, 4:51 pm
That workup assumes every day will be sunny.  If there's one day with
full cloud cover, the batteries will be drawn down to 0 and their
lifetime will be shot.

You should probably triple (at least) that capacity, depending on the
typical weather patterns for the installation.  (How often do you
encounter multiple cloudy days in a row?)

Your panels are also inadequate.  If you really get 500W x 8H, they
can produce up to 4kWH/day.  Assuming no losses, your load is pulling
3.6 kWH/day.  That leaves a scant 0.4 kWH/day for restoring a charge
on the batteries after a cloudy day. You should probably double the
capacity of the panels so that batteries can be fully recharged in one

Posted by angus.berry on January 30, 2009, 4:31 am
Thank you for your response drcsolar. This is exactly the kind of help
I was looking for based on 'ideal conditions', to get a clear example
of calculations. Now I can consider a 'real world' scenario.

Posted by drcsolar@gmail.com on January 30, 2009, 10:26 am
 On Jan 29, 11:31pm, angus.be...@gmail.com wrote:

Yeah truely Ideal, I am an over estimater so i would go three times to
normal within means that is......

Good luck with your project.

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