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HiLonesome
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# Posted: 20 Dec 2014 15:16
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Hi all! First post here after several years of lurking. Believe me, I will have more! I am in the planning stages for a 16 x 24 with 1/3 loft. Right now I'm working on design of the 12v solar system. I tend to get everything thoroughly planned before I lift a finger...besides, it's winter!
I have done a lot of research, including searches here, and built a spreadsheet for my design/sizing. I think I have most of the info in place, but I am missing an understanding of the PV array sizing aspect. I am at about 49 deg. N.
I started with a load estimate (around 900 watt-hrs/day, yes very modest needs). Allowed for 2 days usage with zero sun, 4 hrs of sun/day, and never dipping below 66% of battery capacity. I come up with a needed bank capacity of 452 amp-hrs.
Now what? I am looking at the Renology 100w 12v panels that have such a good rating on Amazon. How do I calculate how many I need? My thanks for your thoughts, what a great resource this site is!
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MtnDon
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# Posted: 20 Dec 2014 15:30 - Edited by: MtnDon
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Quoting: HiLonesome and never dipping below 66% of battery capacity.
Maybe it's just me, but does that mean using a maximum of 66% of the rated capacity or using only 33% maximum and leaving 66% in the pot?? I take that to mean only using 33% max. At least then the 452 makes some sense.
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HiLonesome
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# Posted: 20 Dec 2014 15:33
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What I mean is never using more than one third of the total capacity (leaving 66 percent in the pot).
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MtnDon
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# Posted: 20 Dec 2014 15:52
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To that 452 AH you should apply a correction for the inverter inefficiency. At least 10%, maybe 15%.
Then there is the question of cold weather and how that affects the batteries capacity. EG, if the batteries live at 30 F they lose about 22% of the capacity rated at the STC of 77 F. Applying those would bring theoretical needed battery capacity to around 650 AH.
So nail down the batteries that you would be using before getting into the fine points of panel selection.
FYI, ideally we would want to be able to charge the batteries at a rate equal to 10% of the AH capacity. 65 amps in this example. Common FLA batteries can accept tat much current w/o overheating as a rule and that current rate is high enough to mix the electrolyte as they bubble. It will also help get the batteries back up when the hours of good sun are short.
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HiLonesome
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# Posted: 20 Dec 2014 16:00
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Thanks! Right now I am assuming I will use 2 strings of 2 golf cart batteries, at 225 amp/hrs each. And I am all 12 volt, so will not have an inverter built into the system (might have a very small one using a 12v lighter plug for the rare times I might need 120v).
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