As we have seen, electrical power can be found using P = I × V.
You probably also know that there is another equation involving both current, I and voltage, V: V = I × R
Those of you who know what simultaneous equations are (stuff you do in Maths!) will realise we can combine the two equations above!
Since P = I × V, and yet V = I × R we can put the second relationship in the first:
P = I × V = I × I × R = I² × R