108 and 110 so as to refer to a generic network that can respectively include controllers 108 a and 108 b and networks 110 a and 110 b while on its face calculation of these pv current may appear to be trivial that is not the case it had not been recognized thus far that a matrix which is dependent on the topology of the differential network 110 could be constructed to perform this calculation for example the matrix for differential network 110 a will be different from that of differential network 110 b because of the nature of the topology of the differential network the pv currents e g for elements 102 1 to 102 n can be determined from known and or measured controller parameters within local power controller 108 for example one could use converter output currents for controller 108 a alternatively and for example one could perform current summing via kirchhoff 39 s law at the nodes e g nd 1 to nd n u2212 1 where the inductor currents e g i l 1 to i l n 1 for inductors l 1 to l n u2212 1 are function of the duty cycles which are known within local interface 302 of the control signals applied to the switches within the differential network 110 e g switch pairs sa 1 1 sa 1 2 to sa n u2212 1 1 sa n u2212 1 2 or switches sb 1 1 sb 1 2 to sb n 1 sb n 2 for example an input vector that corresponds to inductor currents i l 1 to i l 1 n 1 for inductors l 1 to l n u2212 1 can be where current i main being the current output from the string to central.