A danger of the Channel Energy ability, when a caster over-extends themselves by channeling too much energy through their body at any one time they will start to suffer a loss of health through exhaustion.
Channeling additional energy into an existing spell (light, shield, etc.) can be used to boost its area of effect or increase its effectiveness. However, casting magic is not as simple as pulling a bow string. Each wizard may only spend the amount of mana through their system equal to their intelligence modifier per round. So a wizard with +4 intelligence can spend up to 4 mana per round (thus more advanced spells with a cost of greater than 4 mana either take more rounds to cast or more casters to complete). Additionally, a HUMAN wizard may only continuously channel arcane energy through their bodies (cast magic directly from their own mana pool) for a number of rounds equal to their constitution modifier X2.
Casting a Channeled Spell Example:
A first level wizard with +4 intelligence modifier casts a level 1 spell: Shield. Though the spell's duration lasts for a number of minutes equal to their level, they can continue sending arcane energy into the spell thus perpetuating the spell's effect for a longer duration or they could also enlarge the spell's effect. By spending the mana needed to cast the spell during the second round, they can double the effective coverage of the spell but it would still only last one minute per level unless extended.
By round:
| Round | Effect | Cost |
|---|---|---|
| Round 1 | Cast Shield Spell (+4 ac for 1 person for 1 min/level) | 1 |
| Round 2 | Extend Spell Shield duration (2 min/level) | 1 |
| Round 3 | Enlarge Spell Shield Spell coverage (2 persons) | 1 |
| Round 4 | Increase Spell Shield protection (+8 protection) | 1 |
By the fourth round, the wizard has created a barrier that lasts longer, protects more people and can take a harder hit, but they will begin suffering chanelling sickness (for humans) after the second round (constitution modifier +1). For rounds 3 & 4 they will need to succeede a fortitude save (DC 10+spell level+excessive rounds). The wizard would roll versus 12 (10+1+1) for round 3 and then versus 13 for round 4.
Channeling Sickness Example:
So if a wizard has +4 intelligence they can spend up to 4 mana in a round on a spell or spells. However, a HUMAN wizard (with a constitution modifier of +1) may only continue to cast magic for a number of consecutive rounds equal to their constitution modifier (x2) (or 2 rounds). At that point the wizard must rest for at least one round or begin to suffer exhaustion.
It is for this reason that many human wizards will invest in magical instruments such as staves, rods, wands and scrolls to enable them to cast spells beyond their body's natural limitations.