We report outdoor experimental results of PV-electrolysis systems using a series of proton exchange membrane (PEM) electrolyzers powered directly either through InGaP/GaAs/Ge based concentrated (750 sun) or Si based non-concentrated (1 sun) PV modules. M power matched conditions, solar to hydrogen (STH) conversion efficiency of 18-21% with a production rate of 0.8-1.0 L/(min-m(module area)(2)) of H-2 was obtained using concentrated PV modules (similar to 28-30% efficiency). Conventional Si module (similar to 17.5% efficiency), on the other hand, generated about 0.3 L/(min-m(module area)(2)) of H-2 (Average) with a STH Average of 9.4%. While scaling up of the electrolyzers by increasing the size of their electrodes resulted in a similar voltage losses due to mismatching of their power point with that of PV modules (when coupled without power electronics), scaling them out in series maximizes H-2 production by utilizing the maximum power available from PV. Unlike, PV-electrolysis operated via power electronics, direct integration of PV modules with electrolyzers leads to effective utilization of PV power, with STH values closer to the theoretical limit of the coupled system (STH = PV module efficiency x Electrolyzer efficiency). The H-2 production rate and STH using a concentrated system with a solar tracker remained constant and high during the measurement period (5-6 h). On the other hand, the production rate as well as STH varied with time on a conventional Si module without a tracker under 1 sun, with a significantly low average H-2 production rate. Keeping cost factor aside, PV-electrolysis under concentration outweighs the performance of non-concentrated PV based system by a factor between 1.5 and 3.0.