International Electrotechnical Commission

Photovoltaic system performance - Part 3: Energy evaluation method - Edition 1.0
 N TS 61724-3

Annotation

This part of IEC 61724, which is a Technical Specification, defines a procedure for measuring and analyzing the energy production of a specific photovoltaic system relative to expected electrical energy production for the same system from actual weather conditions as defined by the stakeholders of the test. The method for predicting the electrical energy production is outside of the scope of this technical specification. The energy production is characterized specifically for times when the system is operating (available); times when the system is not operating (unavailable) are quantified as part of an availability metric.

For best results, this procedure should be used for long-term performance (electrical energy production) testing of photovoltaic systems to evaluate sustained performance of the system over the entire range of operating conditions encountered through the duration of the test (preferably one year). Such an evaluation provides evidence that long-term expectations of system energy production are accurate and covers all environmental effects at the site. In addition, for the year, unavailability of the system (because of either internal or external causes) is quantified, enabling a full assessment of electricity production.

In this procedure, inverter operation and other status indicators of the system are first analyzed to find out whether the system is operating. Times when inverters (or other components) are not operating are characterized as times of unavailability and the associated energy loss is quantified according to the expected energy production during those times. For times when the system is operating, actual photovoltaic system energy produced is measured and compared to the expected energy production for the observed environmental conditions, quantifying the energy performance index, as defined in IEC 61724-1. As a basis for this evaluation, expectations of energy production are developed using a model of the PV system under test that will serve as the guarantee or basis for the evaluation and is agreed upon by all stakeholders of the project. Typically, the model is complex and includes effects of shading and variable efficiency of the array, but the model can also be as simple as a performance ratio, which may be more commonly used for small systems, such as residential systems