Over the past decade, photovoltaic solar energy (PV) has been achieved great developments in terms of performance enhancement, cost reduction and scale of deployment. A wide range of materials, device technologies and architectures have been applied to fabricate higher efficiency and lower cost PV. Consequently, Silicon-based single-junction PV, which are dominating the PV industry, are approaching their fundamental efficiency limit of 29.4%. To overcome this barrier, multi-junction or tandem solar cells, which consist of a thin film wide-bandgap top cell and a silicon bottom cell, are very promising. Recently, record efficiencies of 32.5% and 32.8% were achieved for III-V/Si dual-junction solar cells based on stacking of GaInP and GaAs top cells on silicon heterojunction bottom cells, respectively. However, the performance of PV is frequently measured and evaluated under standard test conditions which are considerably different from outdoor operating conditions. In addition, predicting actual harvesting energy of a PV installation as per site-specific conditions is essential for customer’s point of view to choose suitable PV technologies as well as orientation. We, therefore, propose a model to calculate the daily, monthly and annual energy yield of outdoor Si-based PV installations included commercial panels as well as tandem solar cells operating at arbitrary tilt and orientation. This methodology will be used to evaluate the benefit of potential new PV technologies for outdoor domestic and building integrated applications. The advantage of this method is that the orientation of solar panels and their properties can be adjusted quickly without extra experiments and transferring to other locations can be done easily since it based on a minimum of input parameters. In this presentation, the energy yield calculation of outdoor PV installations for different regions in Belgium and Vietnam will be carried out.