This likely has a double meaning: 1) to respond to the high priest's question and 2) to call himself "I Am," which is what God called himself in the Old Testament.
Here "power" is a metonym that represents God. To sit at the "right hand of God" is a symbolic action of receiving great honor and authority from God. Alternate translation: "he sits in the place of honor beside the all-powerful God" (See: [[rc://en/ta/man/jit/figs-metonymy]] and [[rc://en/ta/man/jit/translate-symaction]])
Here the clouds are described as accompanying Jesus when he returns. Alternate translation: "when he comes down through the clouds in the sky" (See: [[rc://en/ta/man/jit/figs-metaphor]])