<p>This study investigates the heat collection efficiency of flat-plate solar collectors using water-based Fe₃O₄ and Cu nanofluids. The effects of nanofluid mass fraction, particle size, flow rate, and external magnetic field on heat collection efficiency were experimentally analyzed. The results show that within a certain concentration range, the heat collection efficiency of the nanofluids gradually increases with increasing mass fraction and flow rate. However, when the mass fraction reaches 0.5 mass%, the efficiency slightly decreases due to the agglomeration of nanoparticles at high concentrations, which affects stability. Smaller particle sizes lead to higher heat collection efficiency. Compared to conventional heat transfer fluids, the maximum heat collection efficiencies of Fe₃O₄ and Cu nanofluids increased by 12.73% and 17.5%, respectively. Under an external magnetic field, the heat collection efficiency significantly improved. At a magnetic field strength of 100 Gs, the Fe₃O₄ nanofluid exhibited a 9.63% increase in efficiency compared to the non-magnetic field condition.</p>