Volts cannot be directly converted into joules. The energy in joules is equal to the voltage in volts times the electrical charge in coulombs. A coulomb is a fundamental unit of electrical charge, and it is the charge transported by a constant current of 1 ampere in one second.
If the charge flow is 4 coulombs, and there is a supply of 15 volts moving along that flow, then the energy is 60 joules. A joule is a derived amount of energy, work or amount of heat in the International System of Units. It is defined as the work required to move an electrical charge of 1 coulomb through 1 coulomb volt, or the work required to produce 1 watt of power for one second.