It’s a problem that utility spells with a high failure rate, such as identify at 95% fail, is a mere inconvenience to players, since they can sit in a closet with a stack of food and wait for their Pw to recharge until they succeed. This is not really a problem with combat spells, since the penalty for failing to cast the spell correctly is a disadvantage in combat, where time and Pw matter. Some possible ways to address it:

• Large penalties for failing to cast a spell, like a 1% memory loss and 50 hunger.
• Failing to cast a spell locks it for a certain period of time. It would be hard to make this not turn into an even slower grind for utility spells, since the lockout period could just be waited out.
• Make spells never actually failable, but Pw cost is increased proportional to the failure rate, specifically: real Pw = base Pw / success rate. Under this formula, a 15 Pw spell at 95% fail becomes a 300 Pw spell at 0% fail. This means the Pw cost should be exposed in the spellcasting menu rather than failure rate, even though failure rate still needs to be calculated. Maybe use different colors in the spellcasting menu to denote spells you can cast now, spells you can cast by waiting to recover more Pw, spells you can’t cast even at your current maximum Pw, and forgotten spells.