## 1 Introduction  
Prior to installation, the photovoltaic (PV) system provider (PVSP) performs a site survey to determine the suitability of the location and to propose the most appropriate system design. Then, the energy yield (\(E_{sys}\)) prediction report will be provided to the customer. This report allows for the return of investment (ROI) or the payback period to be calculated and correspondingly, the financial capital can be secured [1]. One important element in the \(E_{sys}\) calculation is PV inverter conversion efficiency. For convenience, many PVSP use the maximum (peak) efficiency (\(\eta_{max}\)) value from the inverter datasheet to compute \(E_{sys}\)[2,3]. However, it has to be noted that the \(\eta_{max}\) refers to the maximum achievable conversion efficiency of the dc-ac power converter (or inverter) under the standard test condition (STC) (STCs: Irradiance: 1000 W/m\({}^{2}\), Temperature: 25 \({}^{\circ}\)C, Air Mass: 1.5 AM). In practice, the PV system rarely operates at STC. For example, in a region that experiences the Equatorial climate, the STC condition is achievable only for a limited period of the day. At most times, the system is subjected to a much lower irradiance (\(G\)). Moreover, the operating (module) temperature (\(T\)) is typically in excess of 35 \({}^{\circ}\)C throughout the day [4]. Alternatively, the weighted efficiency--such as the European (\(\eta_{EURO}\)) and the California Energy Commission (\(\eta_{ECE}\)) can be used for \(E_{sys}\) computation. The weighted efficiency is based on the consideration that, on average (time per year), the inverter does not operate at STC. Instead, it is subjected to the continuous variation of irradiance and temperature. Since the power loss of the power (semiconductor) switches inside the inverter is dependent on the current level at which they operate, the efficiency value is \"adjusted\" in order to accommodate for the variation in the operating condition. Furthermore, the efficiency of the inverter is affected by the fixed losses and ohmic losses. The former is normally due to the power consumption of the controller and driver circuits, while the latter is related to the other resistive (I\({}^{2}\)R) losses. Thus, the weighted efficiency is considered to be more representative of the real inverter performance. Despite this advantage, it is obvious that the weighted efficiency is specific to a particular climatic region; therefore, its usage in different meteorological conditions can lead to inaccurate \(E_{sys}\) prediction. For example, an inverter that operates in Europe will not give the same output if it were to be installed in the Equatorial region because the irradiance profiles between the two are markedly different [5]. Correspondingly, if the same value of \(\eta_{EURO}\) is used, the \(E_{sys}\), the ROI and payback period for the latter will be wrongly calculated. Notwithstanding this clear mismatch, it is rather surprising to discover that no work has been undertaken to determine the suitability of \(\eta_{EURO}\) for inverters installed in an Equatorial climate. To date, most PVSP still consider \(\eta_{max}\) or \(\eta_{EURO}\) as the benchmark for efficiency, despite the significant difference in the irradiance and temperature profiles. Perhaps, during the initial growth of the PV industry, the market share of the countries outside Europe was much lower, so providing a separate weighed efficiency number for the inverter was not urgent. However, with the recent rapid growth of PV installations in the Equatorial region, there is a need to have a more accurate prediction of \(E_{sys}\). Thus, it is timely to carry out an investigation on this matter. This work is carried out with two primary objectives. First is to prove the hypothesis that if a PV system is installed in the Equatorial region, the usage of \(\eta_{EURO}\) from the datasheet will not be accurate. In other words, the \(\eta_{EURO}\) needs to be recalculated to reflect the inverter performance under the influence of the Equatorial irradiance profile.