<p>Improving the thermal efficiency of solar-powered water heating systems is essential for enhancing renewable energy utilization and reducing reliance on conventional energy sources. In this context, the use of nanofluid-filled heat pipes represents a promising and innovative approach to overcome the thermal limitations of conventional working fluids. This research investigates the potential enhancement of solar-powered water heating systems by applying nanofluid-filled heat pipes. A nanofluid is prepared by suspending copper and nickel nanoparticles (approximately 20 nm) in ethanol, and its thermophysical properties are measured as a function of temperature to improve thermal efficiency. The thermal performance of a 1000-mm-long copper heat pipe is compared between the prepared copper–nickel/ethanol nanofluid and pure acetone as working fluids under simulated daytime solar radiation. Both working fluids are tested under a filling ratio of 40% for a one-hour testing interval. The results of this study shows that the ethanol nanofluid gives the highest values of thermal efficiencies equal to 45.60–62.80%, which are about 4.52% less than those of acetone at the same experimental conditions. However, the ethanol nanofluid yields a slightly lower overall thermal resistance of 0.074 °C W⁻¹ than that of acetone, which is 0.076 °C W⁻¹. These results indicate that although acetone gives a maximum efficiency of 64.70%, while the efficiency of ethanol nanofluid reaches 58.20%, the latter may offer competitive thermal performance to be potentially advantageous for specific thermal management applications.</p>