You are right, you do not need to send all the data. Or you could send all the data and just read the field that you want to know about. There are different strategies and it all falls under your design. What is given here is jut a suggestion.
I read your arduino code, so as I understand, you are already sending the data. Your data looks like this:
Serial.print(v);
Serial.print("V; ");
Serial.print(i);
Serial.print("A; ");
Serial.print(p);
Serial.print("W; ");
Serial.print("PF= ");
Serial.print((p) / (v * i));
Serial.println();
delay(100);
You are printing the label in the reverse order in most of them? It is not common to do that. Instead, you could do:
print("label:");
print(value);
print(",");
and then at the end of the loop() you add println(), exactly as you are doing before to signal the end of the stream for this loop cycle in your arduino side. In Processing, you read data until your encounter the new line character (aka. \n) and then you process the data by splitting it and assigning it to your Processing variables, exactly as it has been discussed in previous posts.
Kf