Information in the article about the phase cannon maximum output is incorrect. In "Silent Enemy" the following is said about the maximum normal output per cannon:
REED: This, ladies and gentlemen is a phase-modulated energy weapon. It's rated for a maximum power output of five hundred gigajoules. Enterprise was designed to carry three of them. We have one, and it's only a prototype.
TUCKER: Our job is to get it up and running and build two more from scratch. The Captain is taking us back because he thinks this ought to be done by the boys in Spacedock.
The overloads are caused later by the alien sabotage and then replicated by Tucker. This means 500 GJ per shot per cannon. A tenfold increase would mean each cannon outputs 5,000 GJ or 5 terajoules.
The 80 GJ number is given in "Cogenitor", and it is given in context by Reed who is talking to an alien he makes it clear he does not trust.