Depends on how much power you plan to pull from the batteries. Think of the batteries as a bank account and the solar system as your deposits, and your usage as withdrawals. You have to have the deposits equal or exceed the withdrawals.
My rule of thumb for solar is a minimum of 80 watts per battery, 100 watts is better. In more extreme cases 120-140 watt per battery to keep up with the loads may be necessary. I use a portable 100 watt system, it will keep up with my light use, primarily some LED lights and vent fans. If we have to run the furnace much at night I usually have to start the generator to get the batteries topped back up. I can usually get away with 3-4 nights in a row before I have to use the generator to supplement my 100 watts of panels. My 100 watt panels only put out around 6.5 amps max in full sunlight. If you are using 30-50 amp hours of battery capacity a night (I have used close to 100 ah on a very cold night) you won't get enough amps to recover during the day.
Aaron
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