The problem with this logic is that "Alderaan Destruction" power also "blow up a space ship."
You're forgetting that "Alderaan Destruction" power also takes an entire day to recharge. So the fact that the Death Star was firing multiple shots during the Battle of Endor implies that it was using much less power for each shot.
I'll admit to not having seen the movies in a while, but...couldn't the Death Star have had more than one laser?
It had several thousand point defense lasers, but only one primary laser array. Just look at any image, it's pretty huge. Also, you see it fire, and it's the same laser every time.
((Forgetting of course that firing a planet destruction laser at a spaceship would have a much stronger effect that just blowing it up.))
Those thousands of turbolasers are only point defense by merit of the Death Star being so large that ordinary cap ships are basically fighters in comparison.
There's also the aspect of it that Death Star 1 and Death Star 2 were different beasts. Who's to say they didn't increase the power output/rate of fire of the megalaser for the second one?
Here are the weapon stats I found for DSII:
1x Superlaser
15,000x Heavy Turbolaser Batteries
15,000x Turbolaser Batteries
7,500x Laser Cannons
5,000x Ion Cannons
768x Tractor Beams
By way of comparison, an Imperial-II class Star Destroyer has this armament:
8x Octuple Barbette Turbolasers or Ion Cannons
50x Heavy Turbolaser Batteries
50x Turbolaser Batteries
20x Ion Cannons
10x Tractor Beams
So yeah. The superlaser is essentially just a really big toy. In canon it is mentioned that a single Imperial-class is capable of devastating a hostile planet or winning a fight with another capital ship. The DS-II has the conventional armament of a massive fleet of SDs, presumably more than enough to render a planet uninhabitable with simple weight of fire. By all rights a DS should be able to take on a fleet of just about any size and win, solo, without using the superlaser. It's only idiotic flaws and Emperor logic that allows them to be destroyed in suitably heroic fashion.