How is God's omnipotence compatible with His inability to sin/do evil?

Consider a very powerful agent (call him John), who can perform every logically possible action, except for turning on my TV. Due to his nature, John just cannot do that. It seems to me that nobody would call John omnipotent, due to his inability to turn on my TV. Now consider God. According to what I've read about classical Christian theism, it is impossible for God to sin/do evil due to his good divine nature. So how is God omnipotent when John isn't? What is the relevant difference between the 2 cases? Consider another agent (call him Alex). Alex, unlike God, doesn't have a perfectly good nature, so he is able to sin and to do evil, in addition to everything that God can do. It seems like Alex is more omnipotent than God (he is able to perform actions that God cannot), which would make God not omnipotent, since nothing can be more omnipotent than an omnipotent being. I would really like to know then, according to classical Christian theism how come God is omnipotent, while John and Alex are not.