Can Kaisa's ability (which gives power for spells) be used to pay for non-spell costs through recycling, or to pay the energy cost of spells like Retreat?

Ruling: Kaisa's ability only provides power that can be used to pay for spells, not energy. This power cannot be used to pay for units or to pay the energy cost of any card, including spells.

Nuances:

• Kaisa provides power (like recycling a rune), not energy (like exhausting a rune)
• The power Kaisa generates can only be used for spell costs, regardless of when or how it was generated
• Kaisa can be tapped at any time during the turn and the power gets stored in the pool
• Kaisa does not give you a rune or energy; she gives power with a restriction that it can only pay for spells