'But of power' means strength of the mind and faith given to us by God through the Holy Spirit. Power is not wished for, but it’s a gift of the Holy Ghost which He endues a believer with. It is the power from heaven, it achieves far more than we can think or imagine through a believer.
We are limited in our strength so we need the power of God in our lives. True strength does not come by relying on our capabilities but Christ. God’s grace is sufficient for us in times of need because His strength is made perfect in our weakness.