The energy consumed to cool buildings is very elevated and solar gains represent a high percentage of these cooling loads. To minimize the thermal load it is necessary to control external shading systems. This control requires continuous measurement of solar radiation in different locations of the building. However, for such applications the use of conventional irradiance sensors increases the cost and reduces the profitability of the installation. This paper is focused on the development, modeling, and experimental validation of low cost irradiation sensors based on photovoltaic effect in order to reduce the costs of dynamic external shading devices and to improve the profitability of the system. With this proposal, firstly, small commercial photovoltaic cells have been adapted for use as an irradiation measurement device. Subsequently, quasi-stationary and continuous experimental measurements of these silicon cells, facing south and installed horizontally, have been carried out in Jaén (Spain) in 2009 and 2010. Finally, a nonlinear multiparameter function has been developed to evaluate the irradiance using the electric current generated by the cell, cell temperature, ambient temperature, and absolute humidity. A favorable agreement between the model predictions and experimental data has been observed with a coefficient of determination around 0.996 for all cells.