The European Solar Test Installation developed the ESTI-sensor (Ossenbrink and Munzer, Proc. 11th EC Photovoltaic Solar Energy Conf., 1992, pp. 333-336, Ossenbrink and Helmke, German 1000 Roofs Programme Coordination Meeting, January 1994) a low-cost solar irradiance measuring device which in a short period of time has become a well-known device for monitoring and evaluation of PV installations and a new standard for PV power plant monitoring systems. To further lower the cost of a monitoring system, and to increase performance, ESTI proposed a lowest-cost system, where all necessary electronics are laminated within the sensor. For this purpose a single-chip controller was foreseen, which measures both solar cell signals (V-oc and I-sc) and performs the algorithm to determine irradiance and temperature.