A design for the all-copper redox flow battery based on readily available, cost-effective construction materials is presented. The design results in a stack cost of between 70 and 300 $/m2, with the electrolyte costing 60 $/kWh. The area specific resistance of the cell is 1.5 Ω cm2. A maximum round trip energy efficiency of 72% occurs at a current density of 40 mA cm−2, this is similar to values reported for zinc–bromine redox flow batteries. The two major loss processes are attributable to permeation of Cu2+ through the microporous separator at current densities below 50 mA cm−2 and excessive electrode overpotentials at current densities greater than 75 mA cm−2. Two methods to characterise the current efficiency of the electrochemical processes in the cell are reported. A round trip current efficiency of 98% is consistently found when cycling the all-copper battery at a current density of 150 mA cm−2.