The determination of seasonal heating or cooling loads in domestic dwellings would be greatly simplified if the forcing functions of the dwellings, the climatic variables, could be represented using only the steady periodic or deterministic component. The differential equations in the time domain describing heat transfer processes could be transformed to algebraic equations in the frequency domain, which could be easily solved through matrix inversion. Temperatures or loads could then be found through transformation back to the time domain. It has been demonstrated that, for solar process heat systems, the stochastic or non-deterministic component of the ambient temperature can be ignored without greatly affecting energy determinations, although the stochastic component of the solar radiation has to be included to give realistic results. In this paper it is shown that the stochastic component is critical for both solar radiation and ambient temperature when determining heating and cooling loads for domestic dwellings.