Testing of small sinusoidal-inverters for photovoltaic stand-alone systems

A test facility was developed and successfully put into operation, enabling the complete and accurate electrical characterisation of stand-alone inverters. This paper presents a description of the facility and the measurement method, as well as results from two new products of ASP, Laupen, Switzerland with 150 and 250 W(AC) output power and input voltages of 12 and 24 VDC, respectively. Both are sinusoidal-type inverters, producing grid-quality power, suitable for all applications. Maximum efficiencies of 89 and 90% were measured, indicating large improvement in performance during the last 15 years. Both efficiencies closely agree with the manufacturer's specification of 90 and 92%. The output voltage at nominal output power was 213 and 221 VAC, i.e. somewhat too low for the 12 VDC inverter. The standby losses were found to be 1.8 and 3.3 W, respectively. The latter is 32% higher than specified by the manufacturer. Improvements are suggested regarding the reduction of losses and the cooling of the transistors. The question as to whether pulsating current withdrawal from 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 retail prices (excluding Value Added Tax, VAT) of the inverters tested are 1.1 and 0.84 Dollars per Watt output power for the 12 and 24 V inverter, showing a cost reduction of approximately a factor of two over the last 15 years. The results from randomly chosen inverters tested at PSI were approved by the Swiss Federal Inspectorate of Heavy Current.