To get a real answer for this question you have to ask, “What is the cost if the power to the load goes off?” You have to ask this question because solar power is intermittent and not really predictable.
If it is low cost or no cost then you really don’t need batteries at all. You simply run your whatever when the Sun is shining and when the Sun is not shining you just don’t run it.
But as the cost of your whatever not getting power increases then you have to start playing the probabilities game. You have to look at the cost of the solar array which will provide the power that you need regardless of the weather conditions versus the cost of your whatever losing power.
That kind of analysis is extremely complicated. However the problem with probabilities is that you can get the math right but then Mother Nature does not cooperate and delivers less solar power than you think you’re going to get no matter how good your math is. Then your whatever turns off and the true cost of solar energy becomes apparent.
I am sure that the people in charge of designing the solar installations for California worked very hard at getting their probabilities right. However as the recent rolling blackouts in California have shown us, Mother Nature does not care about our probability calculations. There was too much Sun in California causing lots of air conditioners to be run, but at the same time there was too much cloud cover in Arizona where the big solar farms are causing less power to be generated. I wonder what the cost to businesses was because of the resulting rolling blackouts. And I also wonder if anybody died because the power went off.
If you want an actual answer to your question you need to tell us what the cost is of your application going off unexpectedly, how many hours per day your application needs to run, where your solar power array is located, etc. With enough information maybe somebody might be able to actually answer your question. And then just hope Mother Nature cooperates.
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