It's a common question, and we've continually put more and more at each controller. We started with 600w, figuring 600w solar / 12 volts = 50 amps? Well, 2 things changed that.
1) Batteries rarely charge at 12 volts, so we should really be figuring for 13 to 14 volts.
2) Solar panels become less efficient as they get hotter, so 600w of solar might only give us 400-500w of output.
If we recompute with more real world numbers we'd have (800w of solar) - 20% loss for efficiency = 640w / 13.5 volts = 47.4 amps. There is some head room there to go a tick higher... and we did with 960w into this controller.
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