Power and energy are very important in magical works. This applies to both the micro and macrocosm. The more power is put behind a working, the better its chances of success. Likewise, the more energy is brought into a working, the better.
As an example from the real world, a WW2 Tiger tank was a formidable weapon. If it had sufficient fuel it could provide devastating results. If fuel was low or non-existent, its efficiency dropped significantly (even in defensive battles).
Another example comes to mind from Anime, more exactly the Bleach series. The main character of the series is one Ichigo Kurosaki, a teen who ended up fighting the good spiritual fight. I remember other characters of the series speaking of him as having a great spiritual power, meaning he was innately powerful. However, when he was involved in longer lasting battles with powerful opponents his spiritual energy would start to drop at some point because of his immense efforts. The same occurred in the case of his opponents.








