Behavioural thermoregulators leverage environmental temperatures to control body temperature. Habitat thermal quality dictates the difficulty of precise thermoregulation, and the quality of behavioural thermoregulation in turn impacts organism fitness. Comparing body temperature of a thermoregulator with the expectations of a null model allows us to estimate habitat thermal quality and the effect of behavioural thermoregulation on body temperature. Available null models do not account for structure in the thermal environment, or transient body temperatures, and use indices that are not closely related to thermal performance. We present a new null model, using multiple random walks through a realistic thermal environment, that generates a temporally integrated distribution of null body temperatures, and proposes a thermal-performance based metric for quantifying behavioural thermoregulation in the context of organism fitness. We demonstrate our model using a small skink in an Australian tropical open woodland and describe a shift in habitat thermal quality through the day, and a compensatory shift in thermoregulatory effort, and hence the benefit of behavioural thermoregulation. Our new null model provides a temporally integrated, realistic estimate of habitat thermal quality and thermoregulatory effort and benefit.