if u have powersetting 120/100 on weapons and fire 6 arrays (3fore + 3aft) at the same time, ur energy gos to 60/100 (6*-10). that means each array make 60% of the max damage, if it hit.
The first weapon is always free. The drain on six beams is supposed to be 50. According to the devs, that drain is supposed to be static -- unaffected by power-transfer rate. During the firing cycle, the power is deducted, and it's supposed to be instantly refunded when the weapons go on cooldown.
That doesn't appear to be how things work, presently. Beams are somewhat better than turrets (which are apparently even more buggy), but you should still notice more than 50 power drained at times. I have, anyway, the last several times I bothered to test it. (Might have something to do with firing arcs, now that I think about it. A controlled broad side rarely yields unexpected results, but sometimes power levels drop way below the prescribed number, maybe when arc is switched mid-cycle?)
There could be two unrelated bugs here, or one bug affecting two different things. We just don't know. Whether you over-cap or not, power drain seems to work counter to explicit dev intention. When you do over-cap, your power levels will behave in a very odd way -- different, at least, from previous behavior, and in the absence of dev clarification, presumably buggy behavior.
To my knowledge, Nadion Inversion is presently the only ability in the game that gives you its full weapon-power benefit, irrespective of your initial setting. That's kinda sorta an indirect buff to Engineer captains, but it comes with a kinda-sorta indirect nerf to EPS Power Transfer and Emergency to Weapons. (If over-capping is busted, then buffs to weapon power are less valuable than they used to be.)
My observation with EPS is that the drained weapon energy is not refunded instantly after the end of the fring cycle, but is raising back up at the speed of you energy transfer. Thus a beam array with 10 power drain, takes normally 2s to refund the power. Since the delay between each volley is 1s, its that 1s difference which makes the weapon power drop lower and lower and lower eventually. If you use 1x EPS console, your energy transfer is 10/s, thus your drained weapon energy from beam arrays is refunded in just 1s, so there is no loss.
Its harder to theorize about the over-capping, as there is no way to determine its exact effect. But its definitely noticeable.