Depending on the meter configuration, apparent energy is calculated by one of the following methods:
Arithmetical
Multiplication of the RMS voltage and current values.
S = Urms . Irms (true apparent power - this method gives good results above Ib/10)
Vectorial
Quadratic sum of active and reactive powers.
S = P²+ Q² (this method is more precise at low currents)
Note: The arithmetical method is not available on meters configured for 3 wire operation, therefore, the apparent
energy calculation will be done using the vectorial method.