1. Introduction In solar cell desig

1. Introduction In solar cell design and ratings, the conversion efficiency under standard testing conditions (STC) has always been the main figure of merit. However, realistic outdoor operating conditions may deviate to a substantial degree from standard testing conditions. These deviations concern the irradiance level (i.e., light intensity), spectral composition, device temperature and other effects. These deviations can have a pronounced impact on the power generation of solar cells, especially tandem solar cells which are well known to be more sensitive to spectral variations due to current mismatch losses (Faine et al., 1991 and Philipps et al., 2010). In addition, as the top cell filters the incoming irradiance, the bottom cell will receive less power than a single-junction 1-Sun solar cell. As a result, low irradiance losses may become more pronounced when tandem solar cells are used in non-concentrating applications. As solar cells are usually designed for optimum performance under STC, power generation under deviating conditions can be reduced significantly. For large-scale PV applications, the most important measure is the energy yield, which in this paper refers to the total electricity generated in a certain time period (see Appendix A), in a given time period (say 12 months), for which the STC cell efficiency is only a first approximation. A somewhat equivalent measure of actual outdoor performance is the performance ratio (PR), which is the ratio between the actual energy yield of a PV system and the expected energy yield of the PV system based on the STC efficiency of the solar cell (see IEC 61724, 1998 and Appendix A). When developing a novel tandem solar cell, it is important to determine its realistic energy yield and potential PR as early as possible. Currently, tandem solar cells are only used in space or concentrator PV (CPV) applications. However, with the advent of c-Si based tandem solar cells, particularly III–V or perovskite on Si tandem cells (Bailie et al., 2015,Werner et al., 2015, Essig et al., 2015, Derendorf et al., 2013 and Taguchi et al., 2003), tandem solar cells may become viable in 1-Sun or low-concentration PV applications. This could be the next efficiency boost to get to the targeted milestone of 30% efficient 1-Sun PV modules. As of now, very few studies have investigated the possible performance ratios for PV systems using 1-Sun tandem solar cells, and it is still unclear how well they would perform under realistic operating conditions. However, it is extremely important to ensure that these tandem cells achieve high efficiencies not only under standard lab testing conditions, but also under realistic outdoor conditions. Therefore, energy yield calculations are needed.

1. Introduction
In Solar Cell Design and ratings, the conversion efficiency under standard testing conditions (STC) has always been the Main figure of Merit. However, realistic outdoor operating conditions may deviate to a substantial degree from standard testing conditions. These deviations concern the irradiance level (ie, light intensity), spectral composition, device temperature and other effects. These deviations can have a pronounced impact on the power generation of solar cells, especially tandem solar cells which are well known to be more sensitive to spectral variations due to current mismatch losses (Faine et al., 1991 and Philipps et al., 2010). . In addition, as the top cell filters the incoming irradiance, the bottom cell will receive less power than a single-junction 1-Sun solar cell. As a result, low irradiance losses may become more pronounced when tandem solar cells are used in non-concentrating applications. As solar cells are usually designed for optimum performance under STC, power generation under deviating conditions can be reduced significantly. For large-scale PV applications, the most important measure is the energy yield, which in this paper refers to the total electricity generated in a certain time period (see Appendix A), in a given time period (say 12 months), for which. the STC cell efficiency is only a first approximation. A somewhat equivalent measure of actual outdoor performance is the performance ratio (PR), which is the ratio between the actual energy yield of a PV system and the expected energy yield of the PV system based on the STC efficiency of the solar cell (see IEC. 61724, 1998 and Appendix A). When developing a novel tandem solar cell, it is important to determine its realistic energy yield and potential PR as early as possible. Currently, tandem solar cells are only used in space or concentrator PV (CPV) applications. However, with the advent of c-Si based tandem solar cells, particularly III-V or perovskite on Si tandem cells (Bailie et al., 2015, Werner et al., 2015, Essig et al., 2015, Derendorf et al. , 2013 and Taguchi et al., 2003), tandem solar cells may become viable in 1-Sun or low-concentration PV applications. This could be the next efficiency boost to get to the targeted milestone of 30% efficient 1-Sun PV modules. As of now, very few studies have investigated the possible performance ratios for PV systems using 1-Sun tandem solar cells, and it is still unclear how well they would perform under realistic operating conditions. However, it is extremely important to ensure that these tandem cells achieve high efficiencies not only under standard lab testing conditions, but also under realistic outdoor conditions. Therefore, energy yield calculations are needed.