אהיה (21) + אשר (1+300+200 = 501) + אהיה (21) = 543. The spaces are not counted, which is the rule for phrases.
Meaning
The full self-naming at the bush is worth 543, and it shows that gematria works on phrases as readily as on words: the three words are summed as one string, and the doubled name is counted twice because it is written twice. The tradition reads the doubling as the name's two faces — the being that is and the being that will be — and the phrase is one of the few in which a divine name occurs twice in a single breath.