The well-established correlation of Hollands et al. (1976), which is applicable to isothermal boundaries and Rayleigh numbers up to 10(5), underestimates the convective heat loss across the air gap of flat plate solar collectors with tube-and-sheet type absorbers both in normal operation and at stagnation. Two reasons for this discrepancy were identified. (1) The Rayleigh number of the air gap above absorbers with highly selective coatings can be three times as high as the application limit. (2) The absorber is not isothermal during normal operation. Based on a literature study and theoretical considerations the application limit of the correlation was extended to Ra = 3.10(5). By means of an analytically derived correction parameter, the correlation was adapted to non-isothermal boundary conditions. For the cost- and efficiency optimization of flat-plate collectors an accurate analytical model was developed, based on the model of Duffle and Beckman (1991), enhanced by the new correlation for convective heat loss between absorber and cover plate. The model was validated against data of standardized collector tests. It is able to predict both the thermal efficiency and the stagnation temperature within the uncertainty limit of the standardized test method EN 12975-2 (CEN, 2006). (C) 2015 Elsevier Ltd. All rights reserved.