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Old 06-25-2019, 09:49 AM   #1
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Unsure about battery bank

So we plan on being connected to shore power using a 50 amp most of the time. We are running ac only through our 40 ft home. We plan on needing a least 900 watts of solar also, could be up to 1200. My question is how do I figure out how many deep cell 12 v batteries we will need. My father should be helping me with more details on the math, but I just get a little confused with the battery bank. And what our options are there.
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Old 06-25-2019, 09:58 AM   #2
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The capacity of your battery bank relates to how 'long' you wish/need to operate your 'house' (some list of equipment) when not connected to shore power, solar is not available, and without running a generator.

Assuming you have performed an energy audit/budget (http://jdfinley.com/energy-audit-watt/), the remainder of the calculation is pretty simple. Of course, that ignores the fact that it is incredibly hard to know whether two days is needed or five days (or x days)!!
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Old 06-25-2019, 10:47 AM   #3
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Are you planning on running your AC unit with solar and or batteries?
Because 900 watts of solar makes around 40 amps of charging power for about 5 hours per day of good sun, a battery bank of 500 amp hours is about the biggest bank that this 900 watts of solar can charge assuming you want to use 50% of the bank daily.
This 900 plus 500 system cannot even run a tiny 5000 btu AC unit for a few hours a day because one amp of AC current equals about 10 amps of DC current.
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Old 06-25-2019, 10:49 PM   #4
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900w is 75 amps, what is the reason you are dropping it down to 40?
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Old 06-26-2019, 11:40 AM   #5
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Your panels will never produce nameplate capacity. Especially if you are not able to orient them perfectly.

My current solar is flat mounted and the best I have seen is about 1200 watts for a few minutes around noon early summer.

I used 50% of nameplate rating as a ballpark and it worked out pretty well.
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Old 06-26-2019, 11:58 AM   #6
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Indeed, when your meters indicate you're getting 90% of your panel rating it's celebration time! The conditions necessary are a combination of rare and brief,
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Old 06-26-2019, 12:11 PM   #7
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Amps

If you do the amps calculation with 12v you get 75+ amps but most 12v panels put our ~17v which drops the amps calculation, and since your wires/breakers/inverters steal voltage you should expect about 40 amps to come out of the inverter
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Old 06-26-2019, 01:37 PM   #8
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At 12V, each 100W of panelage will contribute ballpark 30Ah per day in excellent insolation conditions.

If you're using the energy that is
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