The Covenant Plasma Pistol fires bolts of  superheated plasma with a core power output of 100-150 kilovolts and 2-3 deca-amps (from the Halo game manual).  This variation in the voltage and current leads to the energy per bolt being between 20 to 45 kilojoules.  The shields of the Spartan battle armor can withstand 7 of these bolts before failing and this plays a big part in the determination of the energies of the other weapons in the game as the plasma pistol is the only weapon to have well defined energy output.