Objection 1. It would seem that the Son is not equal to the Father in power. For it is said (John 5:19): "The Son cannot do anything of Himself but what He seeth the Father doing." But the Father can act of Himself. Therefore the Father's power is greater than the Son's.
Objection 2. Further, greater is the power of him who commands and teaches than of him who obeys and hears. But the Father commands the Son according to John 14:31: "As the Father gave Me commandment so do I." The Father also teaches the Son: "The Father loveth the Son, and showeth Him all things that Himself doth" (John 5:20). Also, the Son hears: "As I hear, so I judge" (John 5:30). Therefore the Father has greater power than the Son.
Objection 3. Further, it belongs to the Father's omnipotence to be able to beget a Son equal to Himself. For Augustine says (Contra Maxim. iii, 7), "Were He unable to beget one equal to Himself, where would be the omnipotence of God the Father?" But the Son cannot beget a Son, as proved above (Question 41, Article 6). Therefore the Son cannot do all that belongs to the Father's omnipotence; and hence He is not equal to Him power.
On the contrary, It is said (John 5:19): "Whatsoever things the Father doth, these the Son also doth in like manner."
I answer that, The Son is necessarily equal to the Father in power. Power of action is a consequence of perfection in nature. In creatures, for instance, we see that the more perfect the nature, the greater power is there for action. Now it was shown above (Article 4) that the very notion of the divine paternity and filiation requires that the Son should be the Father's equal in greatness--that is, in perfection of nature. Hence it follows that the Son is equal to the Father in power; and the same applies to the Holy Ghost in relation to both.
Reply to Objection 1. The words, "the Son cannot of Himself do anything," do not withdraw from the Son any power possessed by the Father, since it is immediately added, "Whatsoever things the Father doth, the Son doth in like manner"; but their meaning is to show that the Son derives His power from the Father, of Whom He receives His nature. Hence, Hilary says (De Trin. ix), "The unity of the divine nature implies that the Son so acts of Himself [per se], that He does not act by Himself [a se]."
Reply to Objection 2. The Father's "showing" and the Son's "hearing" are to be taken in the sense that the Father communicates knowledge to the Son, as He communicates His essence. The command of the Father can be explained in the same sense, as giving Him from eternity knowledge and will to act, by begetting Him. Or, better still, this may be referred to Christ in His human nature.
Reply to Objection 3. As the same essence is paternity in the Father, and filiation in the Son: so by the same power the Father begets, and the Son is begotten. Hence it is clear that the Son can do whatever the Father can do; yet it does not follow that the Son can beget; for to argue thus would imply transition from substance to relation, for generation signifies a divine relation. So the Son has the same omnipotence as the Father, but with another relation; the Father possessing power as "giving" signified when we say that He is able to beget; while the Son possesses the power of "receiving," signified by saying that He can be begotten.
Source: Summa Theologica (New Advent)