Not a huge deal to get a second battery if the existing is in good shape. Do match the type of battery such as deep cycle flooded. Expect the new battery to be worn out when the older battery is done. So if the old battery has four years left that is all you will get out of the new battery. Expect to replace them both together at end of life.
The 12 Volt Side of LifeI bought my primary solar equipment at
https://www.solarblvd.com/About 100 watts is best for storage and for in use consider 200 to 400 watts with two batteries. Need a controller too.
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