This question comes up a lot so I thought I would take a couple of minutes to explain How and Why.
The how is really a simple formula. The first step and most crucial is to determine how many watt hours you will need in a 24 hour period. You then multiply that number by 1.5 which is a fudge factor to account for charge/discharge efficiency, power factor, and inverter efficiency. If no inverter is involved then use 1.3. This will give you your adjusted watt hours per day. So for example let’s say you will use 3000 or 3 Kwh per day. 1.5 x 3000 wh = 4500 or 4.5 Kwh per day.
The second step is to determine how many days of reserve capacity are required. The minimum is 2.5 days and up to 14 days. Then multiply this by 2 (for 50% depth of discharge as you never ever want to discharge more than 50%). So let’s select the minimum of 2.5 days. 2.5 days x 2 = 5 days.
Third step is to take the number of days determined from step two, and multiply by the adjusted daily watt hour usage. So in this example 4500 wh x 5 = 22,500 wh or 22.5 Kwh.
The last step is to dived the watt hour figure in step three by the battery system voltage. So now we must decide on a battery voltage of 12, 12, 36, 48, 60 and so on with multiples of 12 volts. For consumer grade applications the highest voltage (limited by equipment available to the public) is around 48 to 60 volts. Commercial applications can go as high as 500 volts and higher with special exemptions by limiting access to only qualified personnel. The voltage you choose is restricted mainly by the charge controller current capacity vs the solar panel wattage. Basically you want to use as high of voltage as you can afford to minimize power losses on the wring, and to keep the wiring as small as possible to minimize cost. For example the largest charge controller current available today to consumers is 80 amps. These controllers can be used on 12, 24, 36, and 48 volt systems. So a typical MPPT charge controller of 80 amps will have solar panel wattage limitations. For example at:
* 12 Volts max panel wattage = 1000 watts
* 24 Volts = 2000 watts
* 36 Volts = 3000 watts
* 48 volts = 4000 watts
So for this example let’s assume we live in Kansas City which has a winter Sun Hour Insolation of 3.3 Sun Hours. So from another calculation (found elsewhere) we know we have to have minimum solar panel wattage of 1400 watts. So in this example the minimum battery voltage we can use is 24 volts. Now we have all the information we need to determine the battery Amp Hour Capacity needed. The formula is Watt Hour Capacity / Battery Voltage so using the numbers from previous steps AH = 22,500 / 24 volts = 938 Amp Hours.
Fourth step is to select a battery. If possible we want to only have one single string of batteries wired in series to obtain the voltage needed. We only want to use true deep cycle batteries made for renewable energy. This limits manufactures, and you will not find them at Walmart. A very good manufacture with the best warranty is RollsSurrette. They have a very good selection tool you can use. Check the RE battery option, input Desired AH, input +/ Percentage (10 to 15%), and 20 Hour Rate. Then you will see your choices to the right. Select a battery with enough AH capacity to construct with 1 string if possible. In this case the Rolls S1380 is the right choice. It is a 2 volt @ 1050 AH battery so you would need 12 of them wired in series to make 24 volts. The battery carries a 7 year warranty, with 2 years free replacement and last 5 prorated. You can expect 5 years of life out of it with excellent care.
Finally you might be asking why do I need so may batteries and reserve capacity. Well the answer is batteries only have so many cycles (discharge/charge) in them and the number of cycles depends on the depth of discharge. Generically the Cycles vs Depth of Discharges (DOD) look like this:
* 50% = 200 cycles
* 40% = 500 cycles
* 30% = 1000 cycles
* 20% = 2000 cycles
* 10% = 4000 cycles
Hope this helps.
SK
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How Many Batteries & Why Tutorial

Originally posted by Indanao View PostYes, on the Panels. Bought them cracked in shipping by UPS from a Solar Company on Ebay for $660. ......

This discussion is cluttering up a useful sticky thread, so I am closing the thread for now.
If I can figure out how to move the latest posts to another thread, I will, but that ability seems to have vanished in the current version of the software.Leave a comment:

you should probably start your own thread. You have a lot to learn and this sticky thread is not the place for it.
For now plan on 4 or 8 batteries at 12V each or 8 at 6V, 6 will not work.
You can plan on using 9 of the modules or get one more. they should all be tested to make sure they are working.
You will need an MPPT Charge controller like from outback Power or midnight.
you will need an electrician or some serious studying.

We planned a 48V Controler, mmp type.
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No..i guess that would make it a 72V system. I still don't see why not? If you put 2 12V in Parallel you have 12V....many amps.?Leave a comment:

Interesting. Well you could put 2 24V batteries in series with 2 12V Batteries ?
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Yes, on the Panels. Bought them cracked in shipping by UPS from a Solar Company on Ebay for $660. There were 11 on there way to Cebu. Sounds like I may need one more. Not quite sure what CC is...combiner?Leave a comment:

Well, for 4 in series, and 2 in parallel off of two of them. Just for the storage.Leave a comment:

I thought you were building a 48V system?? why would you buy "6" 12V batteries for a 48V system? did you mean 8 or 4?
also you said you are getting 11 modules? bad idea. don't get a prime number of modules. What CC are you using? most likely you are going to want strings of 3 so you would want 12 modules.

Your right. But, the reason it won't start in Cold Weather ( and it never has ), is because it isn't an AGM Battery. An AGM Battery will sustain max CCA ( cold cranking amps) until it dies. I'm guessing then that the answer is  because it won't last.
Just curious as to your personal solar experiences  and if anyone who's tried it can say what to expect. I expect to have to buy 6 12V 250amp Storage Batteries for our 3080W system (11 280W Panels) from China. They quoted me $219 CNF Cebu per piece.Leave a comment:

When I'm in Canada, my LandRover diesel won't start when it gets down to zero degrees. I have run it flat 10 times, ( right dead ) boosted it off my other car, and it came right back. I guess all batteries deteriorate some each time used and recharged.Leave a comment:
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