Type X phasers were referenced in the TM to be in the gigawatt range... but this doesn't really hold ground when we take into account that the average grade photon torpedoes from TOS/TMP era are supposed to be carrying 1.5kg of matter and 1.5kg of antimatter, resulting in 64 megaton explosion (probably shaped charge style) - though conversion efficiency wise, it was indicated that 48 megatons is a closer explosive yield from unofficial sources. Using the photon torpedo as a reference, a Type X phaser would be thousands of times less powerful if the gigawatt ranges hold credibility - which they don't really when compared to what they were doing with them for precision drilling, and how much the power output needed to be reduced. Some have speculated that Type X phasers on the Galaxy class are equivalent to 100 000 TW per second. 135 000 TW would be roughly equivalent to 31 megatons. So, a Type X phaser at maximum setting would deliver same energy as a lower yield photon torpedo in about 2.75 seconds But, if these figures are more appropriate for TNG and beyond... then if the conversion efficiency for photons is above 90% and close to say 60 megatons... then a 2 seconds phaser burst at maximum power could easily be seen as a viable offensive starship weapon.