Conventional open-sea cultivation constrained by environmental factors is singly incompetent to sustain the rising seaweed demand. This necessitates a complementary strategy to reinforce the existing cultivation system and expand the global seaweed industry. Present study proposes cultivation of Ulva lactuca in temperature controlled flat panel photobioreactors under natural illumination. Adaptability of U. lactuca to the flat panel system is apparent through growth studies and photosynthetic performance (F-v/F-m) across individual panels. Evident effect of annual variation in irradiance on daily growth rates, biomass productivity and composition is portrayed. Significance of initial stocking density and harvesting frequency is highlighted. Poultry litter extract was used as an alternative N-source for sustainable cultivation. The maximum achievable productivity was 303 g m(-2) d(-1) (fresh weight) expanding to 910 tons ha(-1) yr(-1) including biomass composition consistent with the control media. The present pilot scale study delivers valuable information for commercial scale photobioreactors for seaweed cultivation.