The ISO 13790 standard presents two quasi-stationary calculation methods for including greenhouse systems in the evaluation of a building’s energy balance – a detailed and a simplified method. These adopt different assumptions with regard to solar gains in sunspace and in adjacent heated rooms; thus, the calculation results of the internal temperature of the sunspace and the energy demand in heated spaces may differ. This work presents the comparison of energy effects achieved due to the sunspace in an example living space, calculated by means of both quasi-stationary methods and more accurate dynamic simulations with an hour time step. Analysis of the results obtained by means of the described methods has enabled the identification of the advisable range of each of the calculation methodologies.