I need help with hildryn

I've been using hildryn. This build but with some changes for less fire rate and more shield and have arcane aegis. ive started to notice sometimes when I use the 4 with aegis gale that my balefire uses only like 50 energy per use but other times its using like 350 per use. is there a reason for this? can I make it 100% consistent because running out of shield after 5 seconds is annoying. I use the raksa kubrow as well.