As the efficiency of silicon (Si) photovoltaic (PV) moves ever closer to the theoretical limit, 2-junction PV becomes increasingly interesting. Since PV cells are tested under standard test conditions (STC), but real world working conditions differ, it is interesting to see how 2-junction PV performs worldwide in different climates. Worldwide spectra where simulated in SMARTS using data from NASA’s Global Land Data Assimilation System (GLDAS), Clouds and the Earth’s Radiant Energy System (CERES) and Socioeconomic Data and Applications Center (SEDAC) and from the Joint Institute for the Study of the Atmosphere and Ocean (JISAO). SMARTS provides only clear-sky spectra, to account for cloudiness, the BRL model is used. The top absorber is 1.72 eV Perovskite and the bottom absorber is 1.12 eV Si. Because Si is an indirect bandgap material, a limiting Efficiency model by Richter et al. is used that takes both Auger and radiative recombinations into account. Because this model relies on variables and constants only given for Si, and the top absorber is a direct bandgap material, the detailed limit model by Shockley and Queisser is used to calculate the performance of Perovskite. The distribution of high and low local yearly average irradiance is overall realistic, except for the north of Africa and the Middle East. Here aerosol optical depth (AOD) values are elevated. The elevated AOD can be explained by dust events, still these areas should be among the ones with the highest irradiance. The AOD effects the blue side of the spectrum mostly. All spectra where normalised using CERES’ irradiance data. The Richter model is thickness dependent. Thus the optimal thickness of Si is determined to find the optimal efficiency. To distinguish between the effects of the top layer and local climate effects on the thickness, firstly the optimal thickness of single junction Si is calculated and analysed. The optimal thickness ranges from 70 to 870 µm globally. The average is 205 µm and the irradiance weighted average is 165 µm. For double junction Perovskite - Si current matching was used to find the optimum. The thickness of Si ranges from 10 to 4500 µm. But only nine locations (of 597) have a thickness above 500 µm. The average thickness is 77 µm and the irradiance weighted average is 81 µm. Locations with a high optimal thickness had a blue-er spectrum than the locations with a low thickness. Using the optimal thickness of single junction Si and current matching, the optimal top-bandgap was determined around the world. The optimal bandgap ranges between 1.55 and 1.76 eV, the average is 1.655 eV and the irradiance weighted average is 1.663 eV. The relation between the top bandgap and optimal Si thickness was determined under STC. High bandgaps had the lowest optimal thicknesses and the thickness increased with decreasing top-bandgap. Alterations in spectrum and temperature where applied as well to find the effect on that relation. Decreasing temperature and airmass both resulted in an increase in optimal thickness per bandgap vise versa. Changing the spectrum has a greater effect than the temperature change.