The introduction of the performance ratio analysis and the illustration of yield trends will make it easier to compare yield measurement results. The performance ratio indicates the actual amount of solar power produced by a module in comparison to the maximum possible power output. Besides the measured solar irradiation, we also account for the STC efficiency determined by us and the performance at differing irradiation levels.

 
The performance ratio certificate includes all of the relevant information on our test results and the tested module. Additionally, we show two diagrams illustrating the weak-light behavior and yield progression during the past period of measurement, as well as information regarding the temperature coefficient. The charts are explained in detail below.

Performance Ratio

1) Ranking

All solar modules that have been involved in testing for a whole year now (meaning they were installed before Jan. 1, 2011) are ranked based on their performance ratio results. Many of the modules show similar performance ratios. Therefore, measurement variables may lead to alterations in the rankings. Weather variations can influence the rankings, too. Nevertheless, a module placed in the top spot is always considerably better than one at the bottom of the list.

2) Manufacturer and module type
This states the company that comes out with a certain module type. That is either the manufacturer itself or a company that has had the modules produced to order by another manufacturer (OEM) but sells them under the mentioned model name.

3) Yield measurement for 2011
This specifies the year of measurement.

4) Color bar
This bar indicates values (performance ratios) between 70 and 100 percent. Two vertical white lines mark the position of the strongest and the weakest module in the test. The specific values are stated next to each line. A black triangle marks the position of the indicated module within the entire range of tested solar modules. The further this marker lies to the right, the better the module performed in the test.

5) Number of tested modules
This specifies how many modules installed at our test site have completed the whole year of testing. This value can be used to evaluate the module’s position within the rankings.

6) Performance Ratio

The performance ratio (PR) indicates the effective yield a module has actually produced in relation to the maximum theoretically yield possible for this module:

The effective yield is simply the energy in kilowatt hours that the module has generated during the whole year. 
The maximum theoretic yield results of the accumulated solar irradiation (that is the amount of energy, which is irradiated from the sun to the module during the year) and considering how much electrical energy the module could generate from this if it was operated always under standard conditions (25°C cell temperature, 1000 W/m² irradiation, AM 1.5 standard spectrum). This yield is calculated with

 The module efficiency is defined as

in which Standard Test Conditions (STC) are postulated, i.e. the irradiation power per area is determined with 1kW/m². Thus, the module efficiency can be calculated as

With this, for the maximum theoretic yield it is obtained:

The theoretical yield possible is thus calculated by multiplying the accumulated solar irradiation per area with the STC power of the module, divided by the STC irradiation power of 1kW/m². The module area does not contribute to the calculation, since it appears in the numerator and denominator of the fraction and is thus omitted by cancellation:

with Isolar = Total solar irradiation per m² for the year
        PSTC = Module output power under STC
        ISTC = STC irradiation, 1kW/m²

If the actual yield produced by a module is indicated with Ereal, then the performance ratio can be calculated by the formula

If they were operated always under STC conditions, then the performance ratio for each module would be 100%. However, this is not the case in reality. Irradiation and temperature not only change, but also the spectrum of the incident sunlight is itself subject to temporal changes. The performance ratio ultimately reveals how well a module behaves under real conditions. It is therefore important for each module, how little the efficiency degrades with weak radiation at 1,000 W/m², how good the temperature coefficient is and how even the spectral sensitivity is over all wavelengths. On the other hand, the environmental conditions change during the course of the year and they are strongly dependent on the location of the module. Therefore, the performance ratio value indicated here is only valid for the Aachen location. An annual radiation of 1267 kWh/m² forms the basis for the evaluation. Different values would result at other locations with different environmental conditions and also the ranking of the modules would change.

Additional graphs for weak light performance and yield

The curve on the left shows the weak light behavior of the module. The irraqdiation power is plotted on the horizontal axis in the range from 100 W/m² and 1,000 W/m², the module efficiency at the respective irradiation power is plotted on the vertical axis, relative to the efficiency under standard radiation of 1,000 W/m². The latter thus defines the 100% mark. Measurement values were taken with irradiation of 100, 200, 400, 700 und 1,000 W/m². The flatter this curve runs, the better the weak light behavior of the module is, that is the less sensitive this module reacts to differences in the radiation intensity. A module with a flatter curve will, therefore, deliver above average yields at the beginning and end of the year. 
The information on the month under the curve indicates when the weak light measurement was performed.

The curve on the right shows the yield of a module in comparison to the average yield of all modules during the year. The months January (1) to December (12) are plotted on the horizontal axis, how the module behaves with the average yield of all modules for each month is indicated on the vertical axis. Positive values mean above average yield, negative values below average. An average module would have a constant line at 0%. The module shown here delivered an above average yield during the winter months, the yield was average during the summer months. This curve is typical for modules made of crystalline silicon.

Underneath the diagram is the information on the temperature coefficient: On the left the value is expressed as a change in percentage of the output per °K increase in temperature, in relation to the STC temperature of 25°C. For example, the module shown here delivers 15.75% less power at a module temperature of 60°C than at 25°C (-0.45 %/K x (60°-25°)K = 15,75%). All modules have a negative temperature coefficient, that is, the output decreases as the temperature increases. 

 
The temperature coefficient is shown graphically on the area on the right as a deviation from the average (circa 0.44 %/K) over all modules. A green bar means that the temperature coefficient is better than average, that is the output decreases less strongly as the temperature increases. The bar is red in the opposite case. The length of the bar gives the information on how far the temperature coefficient deviates from the average.