Hi @alessandro02, welcome to the forum 
For reference, here’s some relevant discussion on the power breakdown there, including efficiency losses for power transfer.
As @SubseaLED mentioned, using a 12V supply limits the thrust of the T200s, but will also reduce the maximum required power for the vehicle to operate. You will still need to consider how you operate the vehicle though, as the maximum current draw for a single 12V-powered T200 at full throttle is ~17 A, so if your vehicle has 6 (or more) thrusters then trying to run all of them at full throttle simultaneously would require >100A of current, which is double what your power system can provide.