A typically offered warranty on solar panels is from 5 to 10 years for workmanship and up to 25 years for performance. However, it is important to know how solar panel performance warranties are structured. Let’s shed some more light on this issue.
1) You have a panel’s rated power Pmax stated as 150W, with a production tolerance of ±5%. 2) Here you can calculate the Initial Warranted Power (IWP). IWP is the typically rated power Pmax, reduced by the manufacturing tolerance in percentage, that is: IWP= Pmax — 5% = 142.5W 3)
Then you will find that the panel has a 90% warranted stated power for the first 10 years, which means that it is actually 90% of initially warranted power or 128.25W. In other words, only if the solar rated power falls below 128.25W for the first 10 years, you have the right to request a replacement. )
Then you have an 80% guarantee of the output power stated for 25 years, which is actually 80% of IWP, or 0.8 * 142.5 = 114W Similarly, if the solar panel rated power falls below 114W under the above mentioned conditions, you have the right to request a replacement. So far, so good. Somehow, we could accept these surprises. This is, however, not the end of the story. What you should look for is whether the money saving term ‘linear performance guarantee’ is used.
What does linear performance guarantee mean?
If a manufacturer has stated that a solar panel has a linear performance guarantee of 90% for the first 10 years, this means that the panel performance will degrade by the same value year after year within that 10-year period — or 1% per year. This is a very good panel! What if you can’t find the “magic” word linear performance guarantee in the datasheet, and the manufacturer has just stated that the panel has only performance guarantee of 90% for the first 10 years?
This could mean that at the end of the first year of operation the performance of the panel might drop significantly to 91% of its guaranteed power, and during the next 9 years, the performance might only drop off 1% until it reaches the guaranteed 90%. Have you noticed the big difference?
In such a case, for the last 9 years you will incur higher cumulative losses compared to the scenario with linear power guarantee for 10 years. Although at first glance there is no difference between the warranty statement with a linear and non-linear guarantee, it is not true.
Be careful while reading solar panel datasheets!
Since most solar modules have a limited power warranty, manufacturers are not likely to offer module replacement. Instead, they would suggest:
- Repairing the faulty PV modules, or
- Compensating the reduced power output by offering enough new solar modules, or
- Refunding you for the decrease in power output by considering the accumulated years of use. For a warranty of 25 years, the annual deduction is typically 4%. Mind that normally the manufacturer will not reimburse your costs for dismantling, transportation or reinstallation of solar modules.
What is more, warranties normally exclude problems resulting from improper installation, any repairs, changes or dismantling performed by non-qualified staff, as well as accidental breakages or abuses, and also lightning strikes and other natural disasters.
Nominal Operating Cell Temperature
As mentioned above, a solar module is usually rated under the Standard Test Conditions that require maintaining the solar cell temperature at 25°C upon measurement. We also already know that solar cell working temperature is very important for the solar panel’s capability to convert solar energy into electricity. The higher the solar cell working temperature, the less the generated solar electricity.
Therefore, in order to squeeze more power from a solar panel, its average cell temperature should be as low as possible.
The main drawback of the STC rating, however, is that it doesn’t “match perfectly” the real working conditions. That is why Nominal Operating cell temperature rating was introduced — to give you a more realistic picture of the solar panels performance in real working conditions, by providing you the average value of solar cell temperature of a given panel.
According to the standard, the Nominal Operating Cell Temperature (NOCT) is defined as the temperature reached by open-circuited cells in a solar panel at Irradiance of 800 W/m² at ambient temperature of 20°C, wind speed of 1 m/s and panel tilt angle of 45°C . What is the average NOCT of the panels currently being sold on the market? For most solar panels, it is about 48°C (118.4°F).
Such panels are considered ‘good’ ones. By taking into account NOCT and the temperature power coefficient of a solar panel, we can estimate the power losses of the panel in regards to the reference cell temperature 25°C (STC conditions).
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If a solar panel has power coefficient of -0.5%/°C and NOCT of 48°C, the expected
power performance degradation of the solar panel would be: 0.5% * (48°C -25°C) = 11.5% or 11.5% lower than the rated power at the STC conditions. 11.5% power degradation is not a low-performance loss! Low-quality solar panels have NOCT above 55°C (131°F).
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The moral of the story about NOCT is: When comparing two panels, the better one will be the panel of lower NOCT rating!
So be sure to pay attention to this parameter when buying or comparing solar panels.