A methodology to estimate PV electrical production from outdoor testing data is presented. It is based on the adjustment of a well known I-V model curve slightly modified and a new maximum power output expression. The method is developed to provide PV module performance parameters for all operating conditions encountered by typical photovoltaic systems. The method, theoretically based, is reasonably simple, yet accurately predicts operating conditions performance. The main advantage of the methodology developed is that it allows photovoltaic users to control the PV modules production based on measurements that can be performed far away from specialized photovoltaic reference laboratories even when standard testing conditions can not be achieved. (c) 2005 Elsevier Ltd. All rights reserved.