Well for that we'd have to do some math using data we don't have; but we can make some assumptions. There being most of a star is a given, but the size of the star is going to be in question.
The most common mass of a star is .29 stellar masses, so 29% the size of Sol, this would be an M type star. Sol puts out 4.5x1026 W, or 450,000,000,000,000 Terawatts. Over the course of an hour, it thus puts out... 387,189,292.54302 Teratons Of TNT
Now obviously the attack doesn't last for an hour, so we can divide that by 60 twice to get how much it puts out in a second bringing us to 107,552.58126195 Teratons of TNT per second. Now we take 25% of that, as energy output is directly related to temperature and that's directly related to mass and surface area and that means at 1/3 the mass you get about 1/4th the output(give or take, the actual math is a lot more complicated but I don't want to do that right now) and we end up with a total yield of the laser being 26,888.1453154875 Teratons of TNT per second of firing.
So instead of... megatons, you have about 27,000 teratons of tnt.
Now keep in mind, it takes 50,000 Teratons Of TNT to completely blow up a plant. Meaning that after firing this thing for two seconds enough energy is put out to blow up Earth. Even if they can only output 1% of this thing over 1ms you still get 2.7 Teratons of output.
This is such a god damn insane weapon that it would obliterate more or less any capital ship of any faction in 40k, and a shitload of them outside of 40k too.
And that's with a small star, the smallest type of star possible. If the stars used were the size of Sol or bigger the energy output increases dramatically!
This thing is only outclassed by the fucking Death Star in terms of raw destructive output in popular media and it's fucking battlefield capable, you can use it while on the surface of the planet. Well, they can anyway, they don't really worry too much about the thermal bloom or radiation emitted by firing it.