Spellthieves and stolen spells reserve

Question

Kind of related to my other spellthief question.

In the Steal spell description, it is mentionned "At any one time, a spellthief can possess a maximum number of stolen spells equal to his class level" and "Alternatively, a spellthief of 4th level or higher can use the stolen spell power to cast any spellthied spell that he knows of the same level or lower"

Assuming a 20th level spellthief that stole four 5th level spell for simplicity's sake, and they all are Cone of cold. If the spellthief would want to cast a 3rd level spellthief spell, say Globe of invisibility, what would happen to his "spell power reserve" ?

I can see two options :

1. he's left with 3 cone of cold and 2 levels of spells that can only be used to fuel his own spells

   or

2. he's left with 3 cone of cold and that's it

Which should be the correct interpretation?

Answer 1

Choice 2 is the official rule. The phrase “of the same spell level or lower” is referring to the spell as a unit, not simply as a reservoir of spell levels. Powering a 3rd-level spell with a 5th-level stolen spell’s power works (5th is sufficient), but wasteful (5th is greater than necessary).

However, given spellthief is a weak class (and it is), seeking simple ways to make them a little more flexible is a good idea: allowing stolen spells to be broken down into constituent levels to power spellthief spells is a great idea. I haven’t thought of that before, but I think I shall be adding it to my spellthief houserules from now on (along with giving all spellthieves the benefits of the trickster variant for free and eliminating the limit on how much stolen spell energy they can hold).