You may cast any spell with the only limitation being that it may not be of a spell level higher than half your caster level rounded up. When you cast a spell using Power Burn, you burn your constitution score, taking a -1 penalty to your constitution per spell level cast. These penalties stack and return at a rate of 2 per day. This penalty can only be removed through a Wish spell. Miracle reduces the penalty by 5. Heal or Limited Wish reduces the penalty by 2. It may seem strange that no PPP is expended, but a character using Power Burn is literally eating up their body as fuel for the spell which they have no skill at casting.