I-V characteristics of bifacial and mono-facial modules in the test roof-top PV field in Jerusalem were monitored simultaneously. Analysis of factors affecting the energy gains due to contribution of bifacial module back is based on time-of-day, daily and monthly energy generation data. Among these factors:diffuse to global radiation ratio, season and time-of-day Sun position. Daily energy gain of an in-field bifacial vs. a mono-facial module at underlying surface albedo ~0.50 varies in the range 5 – 38 %. An evaluated equivalent efficiency of 21 % is based on the measured yearly gain for bifacial solar cells with a front efficiency 18 %. Optimization of the PV field design should increase these values. Keywords : Silicon Solar Cell, Bifacial, PV Module, Monitoring.