Comparison of small inverters for grid-independent photovoltaic systems

A test facility was developed and successfully put into operation, enabling the complete and accurate electrical characterization of stand-alone inverters. This paper presents a description of the facility and the measurement method, as well as results from thirty new products (available in Switzerland) in the range 100 to 1000 W(AC) output power and input voltages of either 12 or 24 VDC. With the exception of seven test pieces, all generate a simple rectangular or trapezoidal output voltage, useful for insensitive ohmic loads. The exceptions are the latest sinusoidal inverters, producing grid-quality power, suitable for all applications. Maximum efficiencies in the range of 85 to 95%, for non-sinusoidal types, and 89 to 95% for sinusoidal, were measured, indicating large improvement in performance during the last 15 years. Compared to former times, the specifications have become more precise, but in some cases still promise too much. However, in a few cases the efficiency was even higher than specified. The output voltage at nominal output power nevertheless often was too low. Except for five inverters, the standby losses were fairly high, reducing unnecessarily the battery's operating time. The question as to whether pulsating current withdrawal from the battery storage, caused by sinusoidal inverters, has a negative impact on the battery's lifetime cannot be answered from the tests performed so far. The specific prices of the inverters tested were from 1 to 2 dollars per Watt output power, with a cost reduction of up to a factor of two for sinusoidal inverters over the last 15 years. The results of randomly chosen inverter tests were approved by the Swiss Federal Inspectorate of Heavy Current.