Efficiency characterization of 26 residential photovoltaic battery storage systems

Abstract

Numerous loss mechanisms contribute to the overall performance of stationary battery storage systems. From an economic and ecological point of view, these systems should be highly efficient. This paper presents the performance characteristics of 26 commercially available residential photovoltaic (PV) battery systems derived from laboratory tests. They were measured according to the efficiency guideline for PV storage systems. Nine AC-coupled and 17 DC-coupled lithium-ion battery systems are compared. Their measured usable energy content varies between 5.8 kWh and 16.7 kWh and is in some cases more than 19 % below the specifications in the data sheets. Besides the usable capacity, the nominal power and the efficiency of the power conversion system are analyzed. DC-coupled PV storage systems are often advertised with inherently higher efficiency compared to AC-coupled systems. However, the comparison shows that they depend on high battery voltages of several hundred volts in order to exploit their efficiency advantages. The most efficient systems achieve average conversion path efficiencies of more than 97 %. In contrast, the values of the least efficient systems evaluated are only 90 %. Furthermore, the paper analyzes the control behavior by comparing the dead and settling times as well as the stationary control deviations of the investigated systems. Differences in the dead time of almost 3 s and in the settling time of more than 13 s can be observed. In addition, the AC, DC and peripheral power consumption in the fully charged and discharged state are evaluated. While individual systems have an outstanding power consumption of less than 4 W in the standby mode, others consume more than 70 W. The paper shows that various systems still have potential for optimization, especially in terms of conversion efficiency and standby losses. When selecting or optimizing a PV battery system, it is important to consider all loss categories to achieve high overall efficiency.