You can calculate joules from volts and coulombs, but you can't convert volts to joules since volt and joule units represent different quantities.
The energy E in joules (J) is equal to the voltage V in volts (V), times the electrical charge Q in coulombs (C):
E(J) = V(V) × Q(C)
joule = volt × coulomb
J = V × C
What is the energy in joules that is consumed in an electrical circuit with voltage supply of 15V and charge flow of 4 coulombs?
E = 15V × 4C = 60J