Photosynthesis is the major energy harvesting process on this planet. The photosystems converts light energy into chemical energy, which is stored in high-energy molecules like adenosine triphosphate (ATP) and nicotinamide adenine dinucleotide phosphate (NADPH). Conventional methods of measuring these energy molecules require plant extraction and therefore are unable to measure their instant changes or in subcellular compartments. Our group has developed three important energy sensors for in planta studies, including sensors for ATP, NADPH and the NADH/NAD+ ratio. These novel sensors allow us to monitor the dynamic changes of these important energy molecules in various subcellular compartments and of various plant tissues, such as leaves, roots, guard cells and pollens. By using these sensors, we have clarified a few important questions in photosynthesis, as well as in chloroplast and mitochondrial bioenergetics. In this talk, I will share with you our voyage of discovery.