That converter puts out 48.0V (not 24 as your title states) at 8A which is 384W. Assuming your batteries are as low as 12V then the input would be 384W / 12V = 32A but it's probably a bit more due to conversion losses. Assume 35A to be safe. Of course when your batteries are at 13.4V then the amperage will be a little lower. For 30A you would want 10AWG. For 40A you would want 8AWG. And this is with high quality marine grade wire rated for 105ºC. 12AWG shouldn't be used for more than 25A. This all assumes fairly short wires (under 6 feet roundtrip). That's all based on the nice Blue Sea wire and fuse chart. Though putting the numbers into their app I get 12AWG. I entered 12V, 32A, 6 feet total wire, 3% voltage drop, 105ºC insulation, single fused wire. But one telling factor is if you enter 90ºC insulation it suggests 10AWG. This means that 12AWG may work but it will likely be very hot. Remember that a wire at 100ºC won't melt the insulation if it's rated for 105ºC but the 100ºC wire will boil water and burn you badly.