A variety of numerical models for calculating the solar irradiance for an inclined surface are described and evaluated using data for Vancouver, B.C., Canada. While all the hourly models have a common approach for calculating the direct component of the solar irradiance there is a variety of methods for calculating the diffuse irradiance based on the portion of the sky hemisphere within the field of view of the surface. A less significant distinction between the models is in the methods used to calculate the amount of radiation received as a result of reflection from adjacent surfaces.