U.S. Department of Energy - Energy Efficiency and Renewable Energy

Solar Energy Technologies Program

PV Performance: Modules

We provide brief overviews of how to rate PV module performance and how to measure PV system reliability.

PV Module Performance Ratings

The peak watt (Wp) 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:

  • Power output. Power (watts) available at the power regulator, specified either as peak power or average power produced during one day.

  • Energy output. The energy (watt-hour or Wh) output. This indicates the amount of energy produced during a certain period of time. The parameters are output per unit of array area (Wh/m2), output per unit of array mass (Wh/kg), and output per unit of array cost (Wh/$).

  • Conversion efficiency. This parameter is defined as "energy output from array" / "energy input from sun" x 100%.

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.

To learn more about PV physics, see: