The exergy budget in a solar collector has thus far been estimated under the condition of steady solar radiation intensity. In reality, solar radiation temporally varies depending on weather conditions. In particular, it remains to be elucidated to what extent cloud cover affects an exergy budget. To develop a method to evaluate the exergy efficiency with consideration of cloud cover, we studied the exergy budget of a three-dimensional compound parabolic concentrator (CPC) by constructing a model representing the heat budget of the CPC and by measuring temperatures at a few points in the CPC under a variety of cloud circumstances. We found reasonable validity in the thermal model through comparisons of the simulation with measurements. The temperature of the collector fluctuated owing to the appearance and disappearance of clouds, and in response, the input and output rates of the exergy temporally varied during cloudy days. By integrating the rates of exergy over a period including noon, we computed net input and output exergies. The exergy efficiency estimated was maximally 12%. The method developed in this study is also useful in quantifying the thermal performance of other types of solar collectors and improving their design. (C) 2013 Elsevier Ltd. All rights reserved.