How Solar Works Battery BANKS (for off-gird)
A battery serves as an energy buffer between the power producing panels and the power consuming load.  Since grid-tied systems feed directly to the grid and do not provide backup power, no battery is needed.  But, off-grid and grid-interactive systems do require batteries.  A common question is “Do I really need batteries if I only want to work during the day?”  The answer, 99% of the time is yes.  In some rare cases special DC pumps can be connected directly to panels, so can lighting.  The reason for needing batteries is that without them the current being produced would have to exactly match the current being consumed, which is not possible with small scale conventional solar power systems.
As a general rule of thumb, we recommend that the Ah rating of a battery bank should be at least half of the panel wattage rating.  For example, a 200W solar kit should have a 100Ah battery bank.  On an average day, a 200W system will take a 100Ah battery from empty to full.  If a smaller battery bank was used, the battery would reach full charge and the panels would be disconnected from the battery by the charge controller before sunset, leaving expensive panels doing nothing.
A bigger battery bank is always better.  Bigger battery banks provide more backup time and the lifespan of the batteries is also improved.  A system with a large battery bank would likely be less deeply discharged than a system with a small battery bank.  If a battery is only discharged 20% each day it will last longer than a battery discharged 50% each day.
To convert battery Amp hours (Ah) to usable Watt hours (Wh) take the nominal voltage of the battery, multiply it by the Ah rating, and multiply that by 50%.  For example, a 100Ah 12V battery has a usable capacity of 600Wh (12V x 100Ah x 50% = 600Wh.)

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