Macbeth is not primarily about power; if the play has one overarching theme, it would be the reversal of natural order, which begins in the human realm (the murders perpetrated by Macbeth, especially the murder of Duncan) and expands to the cosmic realm (as reflected in the imagery).
This does not mean that power is not one of the play's themes; after all, Macbeth attempts to grab power by murdering kill Duncan. It is interesting to see how different characters use the word "power". In Act II, scene 1, Banquo calls upon heaven to restrain the ambitious dreams trigged by the prophecy by the three weird sisters (emphasis added):
Restrain in me the cursed thoughts that nature
Gives way to in repose!
In Act III, scene 1, when Macbeth sends out two murderers to kill Banquo, he uses the word power in a way that contrasts with Banquo's usage (emphasis added):
and though I could
With barefaced power sweep him from my sight
And bid my will avouch it, yet I must not
The power Macbeth has in mind is the opposite of merciful and is an instrument for maintaining his own (political) "power".
In Act IV, scene 1, the apparition apparition conjured up by the three weird sisters also uses the word "power":
Be bloody, bold, and resolute; laugh to scorn
The power of man, for none of woman born
Shall harm Macbeth.
These words encourage Macbeth to continue on the path he has chosen and suggest that there is a power strong than that of man that protects him. (The prophecy that follows says that Macbeth will not be vanquished until Birnam Wood comes to Dunsinane. Both these prophecies are examples of the theme of reversal of natural order mentioned above.)
In Act IV, scene 3, Duncan's son Malcolm tests Macduff by using language that would also fit Macbeth. For example,
Nay, had I power, I should
Pour the sweet milk of concord into hell,
Uproar the universal peace, confound
All unity on earth.
Less interesting in the context of this discussion is the usage of "power" in the sense of "army" or "military forces", e.g. by Ross:
I saw the tyrant's power a-foot
and by Malcolm:
our power is ready
Of course, a discussion on the theme of power need not exclusively look at how the word "power" is used, but in this case, such an analysis shows that the usage of the word "power" reflects the self-image that some characters want to project.