Why CO₂ monitoring matters for indoor air quality, ventilation control, occupancy sensing, digital twins, and precision agriculture. Explore how sensor technologies such as electrochemical, NDIR, and metal oxide semiconductor sensors, can benefit from wireless CO₂ monitoring in retrofit installations where hard wiring would be costly or intrusive. The paper argues that indoor PV-powered wireless CO₂ sensors can remove the maintenance and sustainability problems of batteries while enabling long-life sensing for buildings, homes, care settings, social housing, and controlled growing environments.