Pmp is maximum power in watt
Vmp maximum voltage at maximum power in volt
Imp is maximum current at maximum power in ampere
Voc open circuit voltage in volt
Isc is short circuit current in ampere
Fill Factor = Pmp/(Vosc*Isc) = (Vmp*Imp)/(Voc*Isc)
In single phase energy meter,
Current coil is made up from thick wire having few no. of turns.
Pressure coil is made up from thin wire having more no. of turns.