It's all logic really.
You find out how much energy the beam outputs during one unit of time and then multiply/divide accordingly to find how much energy there is in a full charge, which is equivalent to the energy output by a steady beam over 20 seconds.
Let's take a look at the following example.
A laser outputs 100 Joules over one second of steady fire.
If one were to charge it ten seconds, how many Joules would it output?
The answer is:
e = e1 * t
e = final energy. That is, the energy output by the charged beam.
Where e1 = energy per one unit of time (which is energy over one second in this case)
t = time spend charging
Thus, firing the laser after ten seconds of charging would output 1000 joules.
And for some reason, subscript refuses to work. e1 looks so very ugly and means something. That 1 should be subscript. You'll just have to make the correction in your mind, seeing that the forum is being silly.