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I have 2 100W panels wired in series . Input: 200W, 24V, 8.3A. When sizing this I had divided the 200 by 13.8V yielding 14.5A...so I assumed the 75/15 was sufficient. Someone else suggested I should divide by 12.0 yielding 16.6. which would mean the controller is undersized. Please advise.
You don't say what batteries or settings you have on the MPPT. So some assumptions in my answer.
Output of the MPPT is at charge voltage, not the nominal battery voltage.
Assuming this is on a sailing boat, so shading of panels is likely, cutting output. So you're unlikely to see full panel output.
At full power (and in very strong sun you may exceed full power) you have 200W available from the panel. So based on your 13.8V, 14.5A is correct.
You're more likely to be charging at 14.4V. So actual current may be lower.
In any case if the panels are able to deliver more than the charger max output current, the charger will just restrict output to its max. And if batteries are well charged they will restrict further. There are limits to the amount the panels can exceed the charger capabilities, but this isn't the case here.
