## 2 Review of PV Inverter Efficiency  
### Overview  
Apart from the module, the inverter is the next most crucial and costly component in a PV system. Since the life-cycle of the system is long, i.e., well over 20 years, the effective influence of inverter efficiency has to be factored into the overall system performance. Based on references [2; 3], the energy yield, \(E_{sys}\) can be predicted using\[E_{sys}=P_{army}\cdot PSH\cdot f_{temp}\cdot f_{mm}\cdot f_{dirt}\cdot\eta_{ calble}\cdot\eta_{inv} \tag{1}\]  
In (1), \(P_{army}\) refers to the rated power of the module, \(PSH\) is the peak sun hour of irradiation, \(f_{temp}\) is the losses due to temperature,\(f_{mm}\) is the manufacturer's tolerance or mismatch,\(f_{dirt}\) is the loss which is due to the accumulation of dirt on the panel surface, and \(\eta_{calble}\) is the efficiency of the DC cable that is used to connect the PV panel and the DC side of the inverter. The last term in the equation, i.e., \(\eta_{inv}\) refers to the inverter efficiency. As a rule of thumb, an inverter which is 1% less efficient is quoted to be 10% comparatively cheaper [9].  
Figure 1 depicts the tree diagram of PV inverter efficiency classification. It has two components: the conversion (\(\eta_{inv}\)) and the maximum power point tracking (MPPT) efficiency (\(\eta_{MPPT}\)). The \(\eta_{inv}\) is simply the ratio of the output (ac) power to input (dc) power, and is directly related to the efficiency of converter hardware. On the other hand, the \(\eta_{MPPT}\) reflects the ability of the MPPT algorithm to tracks the maximum point on the \(P\)-\(V\) characteristic curve at any instant. It is the ratio of the actual power that is extracted from the module to the maximum theoretical power that can be achieved from the \(P\)-\(V\) curve. The \(\eta_{MPPT}\) influences the power converter by adjusting its duty cycle so that it always operate at the maximum point for a given temperature and irradiance level. The total efficiency is defined as the multiplication of \(\eta_{inv}\) and \(\eta_{MPPT}\)[10]. This work concentrates on the \(\eta_{inv}\), as the MPPT efficiency has already been extensively covered elsewhere [11].  
To simplify the calculation for \(E_{sys}\), many PVSPs assume \(\eta_{inv}\), equals \(\eta_{max}\)[2]. The \(\eta_{max}\), which is usually given in the datasheet, indicates the maximum reachable efficiency achieved by the inverter, measured using the STC. Typically, the peak power is achieved when the inverter operates at approximately 70%-80% of its rated power. However, the irradiance does not necessary falls into the range where the inverter is at its peak performance. Moreover, the usage of \(\eta_{max}\) is not realistic because the inverter seldom operates at STC during the field operation. Consequently, the application of \(\eta_{max}\) results in an over-rated \(E_{sys}\) value, causing inaccurate prediction of the ROI and the payback period. The weighted efficiency, on the other hand, takes into account the performance of the inverter when subjected to varying irradiance profile. Since the irradiance is directly related to the input power of the inverter, the conversion efficiency varies according to power level at which the inverter operates. Thus, the weighted efficiency is more representative of the inverter performance because it matches the operational condition with the power converter characteristics.  
Figure 1: Tree diagram of PV inverter efficiency [11].