Every Phaser Array Element has a maximum energy output limit, ergo the need for multiple elements. This not only provides wider firing arcs, but provides many emitters to fire out of simultaneously. And with advanced Energy Capacitors / Batteries, who knows how much energy you could have stored before battle to power each individual element within the array. The Defiant seemed to consume their Power Cells relatively fast. But given how powerful the bolts they were firing, I'm not surprised that they wore out the Power Cells so easily and it needed to be replaced. The Series 5 long-range tactical armor unit had a "Condensed Energy Matrix" which which alone could power a fleet of StarShips. Imagine a hybrid Capacitor/Battery that gave out enough energy to make a small meter long torpedo go to warp, have shields, sensors, tractor beams, and energy weapons. That's insane how much energy you could cram into that unit. And I'm sure Voyager scanned the Druoda "War Head" and knows the structural makeup of every part of that projectile. The most valuable part IMO being the incredible small portable power source that is the "Condensed Energy Matrix". That much power, that fits in the size of a meter long tube that can be carried by two crew members is game changing.