The amount of power involved in the effect of the spell, intended as energy per unit time, cannot exceed what the caster can actually produce with their body.

To make an example, if a fit wizard can produce an output of 500 W, each spell he casts cannot exceed that amount of power, so for example no melting 1 ton of iron by snapping a finger.

In the same way a warrior cannot possibly swing his sword on the battlefield for hours straight while continuously chopping enemies in half or a horse cannot gallop 12 hours in a row, the amount of power involved in the effect of the spell, intended as energy per unit time, cannot exceed what the caster can actually produce with their body at every moment.

To make an example, if a fit wizard can produce an output of 500 W, each spell he casts cannot exceed that amount of power, so for example no melting 1 ton of iron by snapping a finger. And casting several of those spells in a row will inevitably lead to fatigue and tiredness.