in order to certify the performance of a box-type solar cooker, thermal performance is evaluated in terms of two figures of merit viz. F-1 and F-2 (IS 13429:2000). The acceptable criterion to make the solar cooker eligible for ISI mark is that the value of F-1 should not be less than 0.12, and that of F-2 should be greater than 0.40. Further, the solar cooker is classified as Grade A if the value of F-1 is determined to be 0.12 or higher: otherwise, the solar cooker is classified as Grade B. Determination of F-1 and F-2 comprises measurements of the temperature of cooking tray, water temperature and ambient air, solar irradiance, and other associated parameters. The effect of accuracy obtained in these measurements assumes significance in the perspective of grading of solar cookers. In the present work, an attempt has been made to analyze the effect of instrumentation on F-1 and F-2, which is normally employed for testing and standardization of solar cookers. The analysis is supported by a large number of outdoor tests conducted on a box type solar cooker under stagnation as well as load test conditions. It is observed that the variation of absolute Value of error in F-1 and F-2 could be in the range of 0.002-0.003 and 0.015-0.033, respectively. This corresponds to errors of the order of 2.5-3.0% in the evaluation of F-1 and 5.0-6.0% in F-2. (C) 2008 Elsevier Ltd. All rights reserved.