Solar Panels at 45.0° North

About battery life... More or less, a proper deep cycle battery's life is "optimum" when cycled in the range of 80% state of charge to 20% state of charge... Note that if you get "2,000" cycles at 80% state of charge (20% discharge), you will get roughly 1,000 cycles at 60% state of charge, and ~500 cycles at 30% state of charge--Sort of made up numbers for example.

So, If you double the size of the battery bank (following the appropriate solar array design rules of thumbs), it will last ~2x as long and cost ~2x as much. From a "cost per AH delivered" point of few, the costs are about the same. One battery bank may cycle for 3 years and the 2x larger bank will cycle for 6 years.

Practically speaking, taking a battery bank below 50% state of charge on a regular basis can be hard on the battery. Also, that "soft limit" of 50% discharge gives you "room" to run your battery bank for more cycles/years... I.e., if you cycle to 80% state of discharge--When the battery has lost 20% of its capacity, it will fail for you. If you discharge to 50% SOC/SOD, then the battery can lose upward of 40-50% of capacity before you have to replace--giving a potentially longer "useful life" (some mfg. call 20% loss of capacity end of life for warranty claims, I have seen others that say 40% loss of capacity).

So, you are left with a question or two. A smaller/cheaper bank that you have to replace every 3-5 years... Or a larger bank that (more money up front--probably need a somewhat larger solar array to properly recharge too) that may last you 6-8+ years.

And, remember that if somebody "kills the battery bank" (leaves the inverter turned on over winter, guests run lights/pump/tv/hair driers)--Your bank will cost 2x to replace.

There is probably no "right" answer--I like to look at the risk/reward of the design. Risk of larger battery bank vs reward of more years between failures vs risks of higher costs of failure.

-Bill "my two cents" B.