IMHO for batteries to be worthwhile, you need to costly grid power or be off-grid completely with no good way to connect to the grid.
There are places where energy is expensive, and they will not credit you for what the excess energy cost you to produce. In that case, you need to use all of your production and one way to do that is to store it in your batteries. For a solar array to produce all your daily energy, it needs to create about six times as much as you use at peak production time and then be able to use that extra production in that day. There is a point where battery energy is worth having. I am guessing that if you can save > $0.30/excess kWh production, a battery may be in your best interest if your rates are much lower than that a battery needs to be justified by backup utilization.
If energy were free, there would be no need for a battery. At what point do you think energy cost would justify a battery. I have retail net metering. I started in October and went negative until March. By the end of the next October, I had a $25 credit. There is no way to buy a battery that would do that without any energy loss. With batteries, you have to charge them before you can discharge them not net-metering. With batteries, you cannot carry July production to Dec. but with net metering, you can.
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