To measure the output performance of PV modules under natural sunlight conditions it is necessary to measure their I-V curves periodically under all possible weather conditions. Our multiplexing system, called PV KLA MUX, is the ideal solution for nearly every climatic zone. It is built completely modular, so that different combinations of modules, electronic Loads (passive or active) or meteorological sensors can be realized. Because the PV-KLA consists of a passive load together with a high speed data acquisition it is possible to measure different types of modules within the same multiplexing system; crystalline modules together with thin film modules and even single cells are possible with our Zero-Volt option, that measures from -0.3V up to the Open-circuit voltage of the test samples. The cabinet is optimized for low and high temperature, while it normally gets a stainless steel hood for preventing direct sunlight. Additionally there are temperature controlled fans as well as cooling and heating aggregates implemented. Robustness in Combination with accuracy leads to a leading equipment for all PV module test facilities. Several options for a customized optimization of the MUX are available, such as additional data logging, complete meteorological sensor station or additional temperature sensors forever module.