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What To Look For On The Solar Panel Specification Sheet – So Over Rated!

Are you under the assumption that the 5kW system you have been quoted is what your system will produce? If so, we need to discuss how to look at important information on the panels’ specification sheet that should have been supplied with your quote, which will provide you with a more realistic expected power output.

To set the scene I do want to touch on the relationship between current, voltage, and resistance; the amount of sunshine hitting the panels and temperature increase of the panels, creates resistance. This is important because it affects how the panels perform, and they do not all perform the same.

To outline, and as I want to explain this as simple as possible, please excuse my non-technical language– Voltage is the driving force of the circuit, current is the electrons being forced around the circuit by the voltage; the rate of the flow of electrons is measured in amperes, which it is this relationship that determines the amount of power that is being produced. This is important because as the panels heat up on your roof, the voltage reduces because the electrons become a little crazy causing resistance, and therefore reducing the power output of the panels.


So we need to know how the temperature or amount of sun may affect your proposed panels, and how to compare between panels from the data on their specifications sheet. We will look at data collected during the standard test conditions (STC), nominal operating cell temperature (NOCT) conditions, and the temperature characteristics to determine what can actually be expected from a typical panel.

Let’s begin with the STC section, figure 1, is where the panel rated output amount (Pmax) will come from in your quote, which is the total amount of panels by the Pmax amount; being the size of your system. These STC conditions are unrealistic, but either way, it provides data to compare and a rated output to go on in specific conditions. These test conditions are ideal with irradiance at 1000W/m2, a panel temperature of 25°C, and an air mass (AM) of 1.5. What is unrealistic here, is that if the sun was shining with all that abundant energy, it is unlikely the panels on your roof would stay at a comfortable 25°C, they are likely to increase to around 20°C hotter than the ambient temperature. So again I draw your attention to as the panels increase in temperature the voltage will start to reduce, reducing the power output of your panel. Don’t be put off by the drop of power output, this information will just allow you to chose the better performing panel, and be informed of what you are actually paying for.

That said let’s check out the LG325/3301C-A5 specifications, in particular, the 325W column as an example.


Figure 1. Electrical properties (STC) from LG325/3301C-A5 specifications


Standard test conditions, in figure 1, show results completed under conditions that are ideal, as outlined in the previous paragraph. To put it bluntly, the lovely 5kW system that you have been quoted is not going to be sitting on your roof in these ideal conditions, therefore unlikely to produce the quoted 5kW amount, it will have variables daily affecting the power output, i.e. cloudy/intermittent sun exposure, angle to the sun and other uncontrollable factors.

Knowing that your system will not produce what it is being quoted at will allow you to make an informed decision, and knowing what to expect, and you may decide to increase the size to include these factors. The more realistic figures, which we will now get into, are from the NOCT section of the same specification sheet.



Figure 2. Electrical properties (NOCT) from LG325/3301C-A5 specifications


The data in the NOCT section shown in figure 2 are a little more realistic with testing conditions of 800W/m2, ambient temperature 20°C, wind speed 1 m/s) being closer to what your panels may output. The Pmax here in these conditions has gone from a 325W rated output from the STC conditions to a 240W Pmax. So if you have been provided with a spec sheet it is worth comparing these two, so you can base your expectations on this NOCT Pmax rated output, not the STC Pmax rated output.

If you would like to do some calculating you could go as far as checking out the temperature characteristics section on the specification sheet, shown in figure 3.



Figure 3. Temperature characteristics from LG325/3301C-A5 specifications


The NOCT in figure 3 shows what the panel reached whilst being tested under the NOCT conditions, outlined in the previous paragraph, here it reached 45°C. Also in figure 3 the Pmax in this section, is also known as the ‘temperature coefficient of Pmax’ we can use these two figures to calculate how much power will be lost for every °C the panel goes over the standard test conditions (STC) of the panel temperature, remember it was tested at 25°C. The Pmax figure from this data is -0.37 %.

If this is confusing you don’t have to do this, but I thought I would put it here in case.

To work out what % of the max power will be lost for every °C over 25°C we can do this:

(NOCT – STC) x Temp Coefficient =

(45°C – 25°C) x 0.37%/°C = -7.4%

So to determine the more realistic output of the 325W from this specification sheet LG325/3301C-A5 we could expect something closer to 325W x 92.6% = 300W instead.

We could even anticipate what impact a 35°C day may have, consider the panels to be 20°C hotter than the 35°C, bringing the panels to 55°C.

(55°C-25°C) x 0.37%/°C = -11.1%, therefore 325W x 88.9% = 289W.


So a good way to think about the temperature coefficient of Pmax is that the lower the figure the less loss you have in relation to the panel temperature, this could be significant over your annual yield, and over the lifetime of your panels.


The Power Output Tolerance or Power Tolerance is shown back in figure 1, denotes how much electrical power the panel can vary from its rated output (in STC conditions), looking something like this 0 ~ + 5%. Panels with the negative at 0% are a better choice than a panel with a power tolerance with a negative number, which means your panel could go below the rated output by this percentage, not just above it. It is becoming more common to find 0% negatives so you can be picky with your panels, don’t just settle.

To show what I mean panels with a Power Tolerance with an expected deviation of 10 ± 10% could mean the rated output of STC Pmax of 100W could be between 90W to 110W, i.e. 0% is better than -10%, don’t you agree? Anything over the rated output power is great, anything under the rated output power not so great.


Another figure to look at is efficiency, however, you will pay more if efficiency is the deciding factor in your panel selection. A good quality panel with a 16% efficiency rating can be a more viable choice than selecting a panel with a higher 19% efficiency, exhibiting a more expensive price tag. To be clear this figure is not the most important, although when I first started out I did focus on this figure a lot, learning over time there are other factors, some discussed here, that also determines a well-performing panel.


So to wrap it up, the size of the system or the rated output of panel that is proposed to you in a quote, are from the unrealistic STC electrical properties section on the panel’s specification sheet. A more realistic figure to look at is in the NOCT section the Maximum Power Pmax (W), this is more real life, or you can take it on, and do the calculation to see how the temperature will affect the rated output. The Power Tolerance commonly on panels today is between 0 and a positive % if it is between a -/+% you can do better, 0 or up is best. Also, ease up on the efficiency expectations quality products have a pretty good efficiency with acceptable price tags, high-efficiency products expect to pay for it. If you are limited for space then you may have to consider the more efficient panels, otherwise, look at the other figures on the specifications to help you select a panel or reject a panel.


The point of this article is don’t be confused or misled by the size you are being offered, variables change the output, and the output is never a constant value over the day, so be mindful when confirming the size of your system. Having this insight will hopefully help make your expectations realistic, you now know how to compare your panels, and you are equipped to select the best panel you have offered to you.


I would like to point out here, very briefly, that if on your quote the inverter is sized at the same size as your solar system, you may question this. The Clean Energy Council suggests the minimum inverter size for a system can be 75% of the array. Some companies will quote you a 5kW array with a 5kW inverter and unless you anticipate increasing panels at a later time, you can have a smaller inverter.

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Melinda Glew