The essential mineral nutrient potassium (K ⁺ ) is the most important inorganic cation for plants and is recognized as a limiting factor for crop yield and quality. Nonetheless, it is only partially understood how K ⁺ contributes to plant productivity. K ⁺ is used as a major active solute to maintain turgor and to drive irreversible and reversible changes in cell volume. K ⁺ also plays an important role in numerous metabolic processes, for example, by serving as an essential cofactor of enzymes. Here, we provide evidence for an additional, previously unrecognized role of K ⁺ in plant growth. By combining diverse experimental approaches with computational cell simulation, we show that K ⁺ circulating in the phloem serves as a decentralized energy storage that can be used to overcome local energy limitations. Posttranslational modification of the phloem-expressed Arabidopsis K ⁺ channel AKT2 taps this “potassium battery,” which then efficiently assists the plasma membrane H ⁺ -ATPase in energizing the transmembrane phloem (re)loading processes.