In open cavity receivers employed in solar paraboloidal dish applications, natural convection occurs and contributes a significant fraction of energy loss. Its characteristics hence need to be clarified so that it can be effectively minimized in order to improve the system efficiency. The investigation of natural convection loss from cavity receivers was undertaken numerically and was validated using the published experimental results for four different receiver geometries. A good agreement between experimental and numerical results was obtained. Furthermore, the numerical results of all receivers were qualitatively comparable to the predictions by other available correlations hitherto, although it was found that each correlation has a limited range of applicability arising from the particular cavity geometry and experimental conditions used to derive it. To address this shortcoming, a new correlation based on the numerical results for three of the above four receivers has been proposed. The correlation employs a new concept of an ensemble cavity length scale, to take into account the combined effects of cavity geometry and inclination. Despite a wide variety of cavity geometries and operating conditions, the proposed correlation predicts approximately 50% of the data within ±20% and 90% of the data within ±50%. This is better than any of the other correlations published to date. The new correlation is also simpler to use than the most accurate of those previously published.