# PV module rating scheme explained

Every PV module has a power rating—listed in watts—that is a basic metric for initial solar facility design. However, the financial assessment of a project should focus upon the energy—power delivered for a measured time (kilowatt hours)—because it is the energy that is bought and sold, not the power. It is a simple calculation to multiply the nameplate power rating with the number of daylight hours in 25 years to obtain a rough estimate of the Lifetime Energy Production (LEP) of a PV module, but the PSI Rating analysis provides distinguishing performance information.

The PSI Rating method uses a model of sunlight and temperature combined with the various factors that impact LEP, and provides a consistent, meaningful way to compare PV modules both between manufacturers, and from the same manufacturer. The PSI Rating is simple to understand as an intuitive bigger-is-better number that is directly related to the 25-year LEP. With the PSI Rating, designers, installers and financers can quickly compare the LEP expected for various PV modules, and combined with pricing information, make a rational decision on the best PV module for a particular application.

This methodology employs a consistent mechanism for comparison of seven key characteristics that affect LEP, and combines them into one number—the PSI PV Module Rating. This provides a comprehensive view of PV modules and how each compares to others. These characteristics of PV modules have already been measured by independent laboratories, or provided by manufacturers, and compiled by agencies such as the California Energy Commission. An eighth characteristic that affects LEP has been identified but is not generally available, and is not presently included in the rating.

To calculate the PSI Rating, the effects of these seven characteristics on LEP for each PV module is determined, and the resulting LEP is then normalized to a hypothetical PV module having ideal characteristics. These essential metrics are described in the sections below, along with an explanation of how each affects the PSI Rating.

*1. Actual tested maximum power vs. advertised*

Generally, PV module manufacturers advertise a power value for each of their products, and that is a primary factor in the design of any solar power system. PV modules can exhibit an actual tested maximum power that is different from the advertised value. The impact of this characteristic on LEP is determined by multiplying the voltage and current at maximum power as provided in the published test reports to determine the Actual Tested Maximum Power, P_{max}. In LEP calculations, P_{max} is used instead of the advertised PV module power. It is essential that a testing lab employs consistent methodology such as the detailed in IEC 61853-2^{1} obtain the actual values.

*2. Negative power tolerance*

In all manufacturing processes, products exhibit a distribution of deviation from the design target. In PV module production, the power tolerance is often published by manufacturers as a range such as ± 3 percent. Higher quality production lines control this variation better and manufacture products with a smaller (tighter) tolerance. A zero negative power tolerance is better for the customer since PV systems assembled from arrays of such PV modules are more likely to meet the overall system design specifications. Many manufacturers realize the significance of this metric, and bin their PV modules with no negative power tolerance: -0/+3 percent. The PSI Rating takes this characteristic into account by subtracting the negative power tolerance from P_{max} calculated above, and then calculating the energy produced.