PV Module Performance Ratings

The peak watt (W_p) rating is determined by measuring the maximum power of a PV module under laboratory conditions of relatively high light level, favorable air mass, and low cell temperature. But these conditions are not typical in the real world. Therefore, we may use a different procedure, known as the NOCT—or normal operating cell temperature—rating. In this procedure, the module first equilibrates with a specified ambient temperature so that maximum power is measured at a nominal operating cell temperature. This NOCT rating results in a lower watt value than the peak-watt rating, but it is probably more realistic.

Neither of these methods is designed to indicate the performance of a solar module under realistic operating conditions. Another technique, the AMPM Standard, involves considering the whole day rather than "peak" sunshine hours. This standard, which seeks to address the practical user's needs, is based on the description of a standard solar global-average day (or a practical global average) in terms of light levels, ambient temperature, and air mass.

Solar arrays are designed to provide specified amounts of electricity under certain conditions. The following factors are usually considered when determining array performance: characterization of solar cell electrical performance, determination of degradation factors related to array design and assembly, conversion of environmental considerations into solar cell operating temperatures, and calculation of array power output capability.

The amount of electricity required may be defined by any one, or a combination, of the following performance criteria:

This last parameter is often given as a power efficiency, equal to "power output from array" / "power input from sun" x 100%. Power is typically given in units of watts (W), and energy is typical in units of watt-hours (Wh). To ensure the consistency and quality of photovoltaic systems and increase consumer confidence in system performance, various groups such as the Institute of Electrical and Electronics Engineers (IEEE) and the American Society for Testing and Materials (ASTM) are working on standards and performance criteria for PV systems.

PV Performance: Systems

Reliability of photovoltaic (PV) arrays is an important factor in the cost of systems and in consumers accepting this technology. The PV cell itself is considered a "solid-state" device with no moving parts, and therefore, it is highly reliable and long-lived. Therefore, reliability of PV usually focuses not on cells, but on modules and systems.

One way to measure reliability is the rate of failure of particular parts. The failure of solar cells mostly involves cell cracking, interconnect failures (resulting in open circuits or short circuits), and increased contact resistance. Module-level failures include glass breakage, electrical insulation breakdown, and various types of encapsulant failures (e.g., delamination).

Fault-tolerant circuit design involves using various redundant features in the circuit to control the effect of partial failure on overall module yield and array power degradation. Degradation can be controlled by dividing the modules into a number of parallel solar cell networks called branch circuits. This type of design can also improve module losses due to broken cells and other circuit failures. Bypass diodes or other corrective measures can mitigate the effects of local cell hot-spots. Replacement of the entire module is a final option in dealing with PV array failures. However, today's component failure rates are low enough that, with multiple-cell interconnects, series/paralleling, and bypass diodes, it is possible to achieve high levels of reliability.- Reference U.S. Department of Energy