Once you hit that range you're destroying small Moons/Planetoids.
And that's not taking into account that they can store up said energies (Most huge facilitates take into account Watt-Hours rather than just every second.) Meaning that in an hour of building up energy, a Star Destroyer is producing 8 Exatons of firepower, at least half of it can go to the weapons. And Star Destroyers can actually convert most of that into firepower. Convert that into Joules and it's basically using 3 Petatons every second. There has never been a stated power output for a Harrower-Class, while the Power outout for an Imperial Star Destroyer is 9.28e+24 Watts.
