When first installed, they were rated for a maximum output of 500 GJ per cannon; the time period over which this energy would be released was not specified, nor was the delay between shots of this yield. When the phase cannons of the NX-01 were first tested, an overload induced by alien sabotage caused them to fire at ten times the expected output, but it's not clear whether the test shot was intended to be a full-power burst. Nonetheless, the ability to produce overloaded bursts was incorporated into their design to improve the ship's offensive capability.
The following year, Lieutenant Reed told his Vissian counterpart that the maximum output of the weapons was 80 gigajoules, but it was clear in the episode that he did not trust the Vissians and was likely giving false information. 
- Star Trek: Enterprise
- ENT "Silent Enemy"
- ENT "Cogenitor"
- ENT "Judgment"