Summary Challenges posed by the intermittency of solar energy source necessitate the integration of solar energy conversion with scalable energy storage systems. The monolithic integration of photoelectrochemical solar energy conversion and electrochemical energy storage offers an efficient and compact approach toward practical solar energy utilization. Here, we present the design principles for and the demonstration of a highly efficient integrated solar flow battery (SFB) device with a record solar-to-output electricity efficiency of 14.1%. Such SFB devices can be configured to perform all the requisite functions from solar energy harvest to electricity redelivery without external bias. Capitalizing on high-efficiency and high-photovoltage tandem III-V photoelectrodes that are properly matched with high-cell-voltage redox flow batteries and carefully designed flow field architecture, we reveal the general design principles for efficient SFBs. These results will enable a highly efficient approach for practical off-grid solar utilization and electrification.