Many building performance applications (energy use, solar gains, thermal comfort, renewable energy systems, daylight, etc.) require reliable representations of boundary conditions - typically in terms of sky luminance distribution models. Nonetheless, most of the meteorological stations only monitor global horizontal irradiance and data such as direct and diffuse horizontal irradiance or illuminance is not available. However, Generation of sky luminance distribution requires information on i) direct and diffuse components of illuminance; ii) luminous efficacy models. Consequently, multiple methods have been proposed in the past to derive from measured global horizontal irradiance data the diffuse fraction (diffuse fraction models) and to derive global horizontal illuminance from global horizontal irradiance data (luminous efficacy models). This thesis thus examines a number of such models in details and potential for alternative modelling techniques and approaches in three parts: Diffuse fraction models: A number of existing methods for the computation of the diffuse fraction were selected. Actual measurements of global and diffuse irradiance were obtained for seven locations in USA and one location in Austria. The measured global irradiance data for these locations were fed to the aforementioned diffuse fraction models. The calculation results were then compared with the corresponding empirical data. Moreover, a new empirical diffuse fraction model based on Vienna data is developed, which performs significantly better than other 7 models for Vienna, Austria. At the end, the best performing model is selected to be used in predicting diffuse horizontal irradiance as an input in generation of sky models. The comparative assessment yielded a number of findings. The relative performance ("ranking") of the models was found to be more or less consistent across the different locations. However, none of the models can be said to be performing wholly satisfactory. The best performing model was Skartveit and Olseth. Regarding the developed model for Vienna, it only has excellent performance for Vienna location. Sky luminance/radiance models: To explore the implications of the sky model selection on the fidelity of simulation results, we used Radiance to compute the indoor illuminance in an existing test space on the rooftop of a university building. Thereby, the aforementioned two sky models were considered. In addition to latter two scenarios, two other scenarios is created using diffuse fraction model in generation of both sky models. A fifth scenario was a sky model generated based on measured values obtained from a sky scanner. Simultaneously, the actual illuminance levels in this room were monitored under different outdoor conditions (clear, intermediate, overcast). The comparison of the measurement results with multiple model prediction results facilitates an empirically based evaluation of the reliability of outdoor and indoor illuminance predictions in the face of different assumptions pertaining to the prevailing boundary conditions. Luminous efficacy models: Four luminous efficacy models were selected and their performance were evaluated in generating global horizontal illuminance. Results indicated superior performance of all models in generation of global horizontal illuminance from measured global horizontal irradiance.