Creatures have far more power than they can effectively use, because most of their power is tied up with maintaining life, sustaining their production of power. Sometimes a person will dive deep into this reserve to accomplish some otherwise superhuman feat- but there are consiquences, as they will need to recover before they are capable of the sme everyday activity they would normally do. (comparable to strains, broken bones, ect) The particularly disciplined have been known to use this to comit suicide, throwing every bit of their power in one defiant spell, leaving nothing to survive. This total energy is relatively equal for most mortals, and is the basis for the standard unit of magical power- a Soul. Most spells worth mentioning -as spells- measure their mana usage in multiple Souls. A typical healthy person might have between 0.9 to 1.1 souls.

Charging Leystone with even a single soul of power takes months, if you dont injure the donor. More donors can speed the process a bit, so the Empire distributes leystone to every household in the empire, with the expectation that it will be collected, fully charged, when taxes are collected.

The empire looks poorly on warlocks who forcefully drain sentients of their Soul of magic- even from a coldly imperial viewpoint, it reduces the number of taxpaying pesants and runs into diminishing returns in the long run.

Animals have magical energy as well, but are lessconisistnt- a snail would have very litle magic to support it's activity- whereas a full dragon needs more souls of magic just to regenerate magic fast enough to manage it's physical activity.

While I'm posting, for my own reference:
Tovera's Personal spells:
Lightning wand, (Rote use- bolt of lightning, various strengths, limited total charge)
Fireblade (Rote use- Superheated Vorpal blade that does not melt itself- duration limited) (Half a soul to activate or (quickly) deactivate, and a soul every half hour
Philosophy of Manipulation of Ice and Cold (but no creation of ice without existing water)
Philosophy of Air Control and Airborn Maneuver (using wings or Glider as foci, both for mobility and air manipulation)
Philosophy of Unseen Force (superstrength, armored deflection, gross telekinesis)
Philosophy of Enhanced Presence (Intimidation, magical oratory and leadership)
Philosophy of the Long Blade (swordplay)

Tovera's Equipment:
Fireblade (Medium leystone- 15 souls)
Lightning Wand (Wand leystone shard- 10 souls)
Breastplate (Medium leystone- 15 souls)
2 gauntlets (small leystone- 5 souls each)
(personal soul- 1.45 after training)
Armor and helm
Personal Glider (Large leystone- 50 souls)