## 1 Introduction  
Photovoltaic (PV) energy generation will dominate the future energy mix [1]. Thus, quantitative information about the long term performance of photovoltaic (PV) systems is important for many stakeholders, including researchers, companies, integrators, regulators, and investors. The performance of PV systems is influenced by many factors, such as local climatological circumstances, system components, installation, cost, and other miscellaneous factors [2]. In this work, we present the performance evolution of fifteen photovoltaic (PV) systems of six different PV module technologies in three different climates. Changes in performance are presented using the performance ratio (PR) and the performance loss rate (PLR). PV technologies applied in these PV plants are amorphous silicon (a-Si) solar cells (one system), heterojunction with intrinsic thin layer (HIT) hybrid silicon (one system), copper indium gallium selenide (CIGS, one system), monocrystalline silicon (mono-Si, three systems), cadmium telluride (CdTe, three systems), and polycrystalline silicon (p-Si, six systems). The PV systems are located in three different climates, namely, Alice Springs, Australia (arid, desert, hot), Cirata and Pekanbaru, Indonesia (tropical, rainforest), and Bolzano, Italy (temperate, dry summer, hot summer). Data from all PV systems were obtained directly from their monitoring systems. Table 1 shows the systems' features, including their PV technologies and locations. The performance of PV systems can change during their operation. As such, the assessment of the time-series of their monitoring data can be useful to get an insight into their long-term energy yield, reliability, and hence, financial feasibility. Performance ratio has been widely used as a performance indicator of PV systems for more than 25 years. Using PR, one can quantify and compare the quality of different PV systems or module technologies operating at various climate conditions [3]. Performance ratio, PR, is the ratio of the actual and theoretically possible energy outputs. PR measures how effectively the plant converts the sunlight collected by the PV panels into AC energy delivered to the off-taker, relative to what would be expected from the panel nameplate rating. The calculation of PR, therefore, quantifies the overall effect of losses due to inverter inefficiency, wiring, cell mismatch, elevated PV module temperature, reflection from the module front surface, soiling, system downtime, shading, and component failures. Several PV systems installed in different climates can be relatively compared using their PR values because PR is independent of location, the orientation of a PV array, and the incident solar irradiance. The result of the PR calculation is a percentage or decimal value. The present range of typical PR values of different PV technologies is 80-90% [4]. However, rather than being constant, the power of PV systems usually follows a negative trend over time due to a decline in module and system performance. To capture this trend, the performance loss rate (PLR) is calculated. Understanding system performance losses over time is crucial to minimize investment risk because a higher PLR translates directly into a lower amount of energy generation and, therefore, reduces future cash flows [5]. Despite the differences in the assessment methods, the rate of performance loss is influenced by location, design, and operation [6]. The accuracy of the estimations of these indicators depends on data availability, accuracy, and resolution, as well as the methods used in the calculation. The PLR, sometimes also erroneously called degradation rate, is a measure used in the PV industry to quantify a rate of change, with a negative rate representing a decrease in performance [7]. The term PLR is more accurate as it combines actual degradation mechanisms with reversible and irreversible performance loss factors both on module and system level. Module degradation refers to appearing degradation modes such as cell cracks, delamination, or hot spots.