These words are used interchangeably to mean "indispensable or requisite for the fulfillment or accomplishment of something." What is necessary or essential cannot be done without, cannot be dispensed with: "Water is necessary (or essential) to sustain life." "It is essential (or necessary) that you vote today." Essential is the stronger word: it means "pertaining to the very essence of being." If you tell someone that he must attend a meeting, that it is necessary for him to be there, you are expressing strong desire and are implying some compulsion. But if you tell him that it is essential for him to attend, the implication is that if he doesn't, the meeting will be canceled.
Dictionary of problem words and expressions. Harry Shaw. 1975.