4x the power is output if the load impedance is unchanged and voltage doubled assuming no heat loss. The example assumes a resistive load with no protection.
120v at 6ohm load
I=E/R 120v/6ohm= 20A P=ExI 120v x 20A= 2400w
240v at 6ohm load
I=E/R 240v/6ohm= 40A P=ExI 240v x 40A= 9600w
Thats why it’s a big no no to use a 120v recept (5-20R) with 240v. Plug something in that needs 120v normally and a fire can occur.