From [Wookiepedia][1]:

> The power of the first Death Star's superlaser was estimated to have
> been more than 2.4×1032 watts, with an optimum range of
> 2,000,000 kilometers and a working range of 420,000,000 kilometers.
> Powerful enough to destroy a terrestrial planet, it was the most
> powerful energy weapon ever built at the time


And while the original source is not linked on wookiepedia I believe this is from [The Essential Guide to Weapons and Technology][2]


  [1]: http://starwars.wikia.com/wiki/Superlaser#Characteristics
  [2]: http://starwars.wikia.com/wiki/The_Essential_Guide_to_Weapons_and_Technology