The amount of energy in a user. The maximum amount of mana a caster has is equal to seven times their force rating. The amount of Mana consumed depends on the spell and force of the spell. If a spell can be sustained it will drain additional mana based on force, if unable, the spell ends.
It takes one second to transfer one mana, and after charging a device, mana then recovers at the rate of one unit per second. Transferring mana causes fatigue. Magic users practice tracking their mana pool so they know how much they can transfer without passing out and learn to count seconds between casting.
Waiting for the number of seconds equal to the spell force between casting is a common practice to reduce fatigue.
Reaching mana zero will cause pain and fatigue to the caster.