Passive radiative cooling technology has the potential to revolutionize the way of cooling buildings and devices, while also helping to reduce the carbon footprint and energy consumption. Pioneer works involving anodic aluminum oxide (AAO) nanostructures showed controversial results. In this work, we clarify how the morphological properties and chemical structure of AAO-Al samples affect their optical properties and their cooling performance. Changes in thickness, interpore distance, and porosity of the alumina layer, as well as the used counterions, significantly affect the cooling ability of the AAO-Al structure. We measure a maximum temperature reduction, ΔT, of 8.0 °C under direct sunlight on a summer day in Spain, coinciding with a calculated peak cooling power, P cool, of 175 W/m2, using an AAO-Al sample anodized in sulfuric acid, with 12 μm of AAO thickness and 10% of porosity. These results represent a significant improvement over previous studies, demonstrating the potential of AAO nanostructures to be used in thermal management applications.