This study extends the applicability of the analytical criterion developed for estimating optimum channel geometry of conventional solar air heaters (SAHs) to other flat plate types. Assessment is made for two other basic types with flow over the absorber and on both sides of it. Heater performance characteristics are analytically determined over a wide range of channel depth-to-length ratios (D/L) and are compared with those of the conventional type, having a flow channel under the absorber. The effects of air mass flow rate, emissivity of channel plates and wind heat transfer coefficient on the accuracy of the criterion are also investigated. For these two SAHs, the criterion is found to be equally valid as it predicts the optimal (D/L) ratio that yields best performance. Further, the ratio of D/L = 2.5 x 10(-3) represents the criterion simplified version for variable flow operation.