The study presents a remarkably simple criterion for determining the channel optimum depth-to-length ratio (D/L)(opt) which effectively maximizes the useful energy from collectors that are designed to heat a fixed mass rate of air flow. The engineering accuracy of the analytically derived criterion has been corroborated over a wide range of ambient variables, design parameters and operating conditions. The results demonstrate clearly the decisive role of the (D/L) ratio in enhancing heater performance. A poor choice of this ratio can significantly affect the thermal conversion efficiency, especially for lower specific mass rates. In contrast, decreasing the (D/L) ratio beyond the optimal value leads to a slight enhancement in heater efficiency. Of significance is the finding that, by choosing (D/L)(opt), we maximize also the air temperature rise. For variable flow operation, however, the ratio of D/L = 2.5 x 10(-3) is recommended as the optimal. Moreover, the reported results and discussions have provided valuable conceptual insights regarding generalization of the performance curves of conventional solar air heaters. It is found that the analytically determined channel depth parameter, in conjunction with the air specific mass rate, are quite successful in correlating the performance in general terms.