Another way to look at power is to compare it to (real-life) water:
Power Capacity (capacitors) is like a pool and Power generation (reactors and aux) are like a hose feeding the pool.
So everything that requires power pulls water out of the metaphorical pool. If you have something steady, like a radar jammer, that's pulling 30 gallons out of the pool every minute, you'll need to be sending enough water through the hose to refill the pool at least 30 gallons a minute for perma-jamming. (replacing what is used)
For a missile example, a missile dumps 3/4 of the entire pool contents at once, but only fires once a minute or so, so you'll need to double the size of the pool with more capacitors, in order to fire two at once. To maintain sustained fire, you'll want enough water coming through the hose to refill the entire pool every minute or so. Replacing what you use is the same thing here. It's just that with big, one-shot, long-recharge weapons, you have some "downtime" between when the pool is emptied, to fill it again.