The top heat losses that constitute the major losses from the box type solar cooker have a strong influence on the thermal performance. To predict or evaluate the thermal performance of a cooker, the top heat loss coefficient U-tw for a water loaded cooker must be known. In the present study, several indoor and outdoor experiments were performed on a solar cooker to develop a correlation for the variable U-tw as a function of pot water temperature, wind speed and ambient air temperature. The proposed correlation for U-tw is employed to obtain heating characteristic curves or predict the thermal behaviour of the cooker for different quantities of water, solar radiation levels and wind speeds. The investigations reveal that the pot water requires less time to reach a certain temperature with an increase in solar radiation level, while it takes a longer time with higher values of load of water in the pots, as expected. However, the effect of wind speed on the time required for certain rise in pot water temperature is marginal. (C) 2003 Elsevier Ltd. All rights reserved.