Why was only part of the Death Star's power used?

Question

In Rogue One we see

the Death Star fire in "single reactor" mode at the Scarif to destroy the Imperial archives.

The main question is

why not just blow it all up? You'd also take out the Rebel fleet, most likely.

Answer 1

I can think of a few reasons:

• They didn't need to. Single-reactor shots are proven effective at destroying targets the size of the thing they wanted to destroy.
• They didn't want to wait. It seems to me like it takes longer to prepare a full blast, but I have not yet had the chance to compare the scenes from A New Hope and Rogue One. Anyway, if true, it was important to destroy the station quickly to prevent further data loss.
• A full blast was untested. They hadn't done a full test of the weapon yet, but as I mentioned in my previous point time was of the essence. Better to leave the full test for later, against a peaceful planet with no rebels (or Empire ships) nearby.
• The Rebel fleet was as close as the Death Star was. So either they both get blown up or neither of them do.
• We don't know what's on the rest of the planet. There may be Empire assets on the other hemisphere that are not compromised. Honestly, I don't see why you'd shield an entire planet for a single building.
• You can still call it an accident. They covered up the other test firing as a mining accident. That wouldn't work the second time if the planet was gone.

Answer 2

The simple answer is that they likely couldn't.

You will recall that the Death Star wasn't fully operational until partway through Episode 4.

Princess Leia Organa: Governor Tarkin, I should have expected to find you holding Vader's leash. I recognized your foul stench when I was brought on board.
Governor Tarkin: Charming to the last. You don't know how hard I found it, signing the order to terminate your life.
Princess Leia Organa: I'm surprised that you had the courage to take the responsibility yourself.
Governor Tarkin: Princess Leia, before your execution, I'd like you to join me for a ceremony that will make this battle station operational. No star system will dare oppose the Emperor now.
Princess Leia Organa: The more you tighten your grip, Tarkin, the more star systems will slip through your fingers.
Governor Tarkin: Not after we demonstrate the capabilities of this station. ...
General Tagge: Until this battle station is fully operational, we are vulnerable. The Rebel Alliance is too well equipped, they're more dangerous than you realize.
Admiral Motti: Dangerous to your starfleet, Commander; not to this battle station.

However, as Werrf mentioned, the likelihood of destroying oneself when hovering a few thousand miles away from a planet you intend to utterly destroy in an explosion ten times its size... is fairly high.

Answer 3

If you recall, there was a fair amount of hand-wringing over its initial use; they had to come up with a cover story for the

destroyed Holy city on Jedha,

because they weren't ready for the rest of the empire to know about it yet.

Answer 4

You would also likely take out the Death Star, and the Devastator. You don't drop a strategic nuke in the middle of your own fleet if you can avoid it.