24V amps
At 24 volts, amps are watts divided by twenty-four. A 240 W load draws 10 A: exactly half what the same load would draw at 12 V, which is the whole reason larger off-grid systems use 24 or 48 volts.
How to work out 24 V current
Doubling the system voltage halves the current, which quarters the resistive loss in the cable, because loss goes with the square of current. That is the single strongest argument for 24 V over 12 V in anything but the smallest installation. The counterweight is component availability: 12 V accessories are everywhere and 24 V ones are a specialist purchase, so many systems end up with a 24 V bank and a DC-DC converter feeding a 12 V branch for the accessories.
Questions
10 A. The same 240 W at 12 V would be 20 A.
Half the current for the same power, and a quarter of the cable loss. Cable gets thinner and cheaper.
Only through a DC-DC converter. Connecting them directly destroys them.
Less of it, but voltage drop still matters; a 1 V drop is 4% at 24 V, which is significant.
For anything over a couple of kilowatts, yes. The same logic applies again, which is why home battery systems are almost all 48 V.