The flexible production of biogas makes it a promising candidate to become the first renewable, on-demand energy source. One appealing possibility to achieve an on-demand supply of methane is to adjust the output of the biogas plant itself. As a consequence, the biological process will be partially overloaded and might fail. Hence, a prerequisite to avoid cost-intensive failure is a fast, reliable, and cost-efficient analytical technology for quickly determining the biogas quality. A low-cost, robust, in-situ online measurement system for the real-time determination of carbon dioxide and methane concentrations is presented. The approach makes use of photoacoustic detectors, allowing for the development of gas sensors with reduced size that do not show cross-sensitivities towards water vapor. The system's readings may be used as input for a novel type of control system that enables an active, demand-driven control of actuators, such as feeding machinery or mixers.