This work presents a novel analytical approximation of the effect of inter-row shading on large photovoltaic (PV) arrays. Computation time is reduced orders of magnitude relative to full numerical simulations, allowing this method to be used in annual production estimation software - for instance, it is currently implemented in the National Renewable Energy Laboratory's System Advisor Model program. A further advantage of the analytical approach is that, unlike numerical simulations, computation time does not increase with the size of the PV installation. Comparisons with full I-V curve simulations indicate that this simplified approach has typical error of 1% over multiple module fill factor, shade extent and shade opacity assumptions. Maximum error of 2-6% was found for simulations of crystalline silicon modules. Comparisons with experimental results show good agreement between the experiment and the model over a range of operating conditions, and intercomparison with prior modeling methods indicates a spread of possible model results, depending on model assumptions. (C) 2013 Elsevier Ltd. All rights reserved.