Does Kai'Sa Daughter of the Void give floating energy or floating power?

Kai'Sa, Daughter of the Void gives floating power, not energy.

According to RiftJudge FAQ #2634 and #5342, Kai'Sa's ability specifically adds power to your rune pool. This power is restricted and can only be used to pay for the power costs of spells; it cannot be used to pay for energy costs (the numbered cost in the top corner of a card) or to pay for unit costs.

Additionally, as clarified in RiftJudge FAQ #8961, this power can be "floated" (retained) in your rune pool until the end of your turn, allowing you to accumulate it for later use.