The best Information I could find is this:
http://www.reddit.com/r/Starmade/comments/1heb1m/so_i_did_some_theory_crafting_with_the_antimatter/
In the comments this also hidden..:
\"From my quick test of it, it appeared that each block used 10 power when firing\"
So if you build one prototype of a single cannon, you could read (with T) the reload/firing rate and thus calculate how much energy you would need.