Understanding Solar Panels – The Solar Panel Characteristics Guide

Solar Panel Characteristics - Solar Photovoltaic

Like any other electrical component, Solar Photovoltaic (PV) Panels have key electrical characteristics that are defined by the materials that make it. These electrical characteristics describe how voltage and current vary for each different type of Solar Panel.

In this guide we will describe what Solar Panels are and help you to understand the relevant data and acronyms so you can confidently choose suitable panels for your Solar PV array.

Key Characteristics

When you look at the Datasheet or Website for any Solar Panel, you will notice four key characteristics that are used to for any and every Solar Panel to help you understand it’s performance under certain conditions.

We have outlined these below, just press the “+” icon to expand the accordion and learn more!

The Voc or “Open Circuit Voltage” is the maximum voltage that a Solar PV panel can output. This parameter is very important when designing a system because it can be used as an indicator of what each panel can contribute to the voltage of the string. 

For strings wired in series, the Voc multiplies per panel. Therefore, it is crucial to pay attention to this characteristic to ensure your string remains comfortably in the voltage range of your inverter.

Vmpp is the voltage that would be expected from each Solar Panel if the panels are outputting at their maximum power point. Your panels are connected to an MPPT (Maximum Power Point Tracker) in your solar inverter which will typically attempt to achieve this Vmpp figure.

You’ll notice that this figure is usually lower than the Voc, and that is why it is not crucial for sizing. It represents the likely operating voltage rather than the highest voltage, so assuming the Voc is within range then the panels will not exceed the limits of the inverter.

Isc refers to the maximum theoretical Short Circuit Current output of the panels, measured in Amps. This figure is also used to ensure that the string of panels will operate within the safe boundaries of the inverter it’s connected to.

Impp is similar to Vmpp, but applies to the current at the Maximum Power Point instead. This means it is the current that would be expected from each Solar Panel if the panels are outputting at their maximum power point. 

What is the meaning STC and NOCT and how does it effect the Solar Panel?

Now that we understand the key characteristics of a Solar Panel datasheet, there are a couple of acronyms you will notice that can help us analyse the panel further.

All of the characteristics above are given based on STC, or “Standard Test Conditions.” This is important to keep in mind because the characteristics of the panel will change as these conditions change – the datasheet is just giving an overview at pre-agreed conditions to allow panels to be compared.

STC - Standard Test Conditions

The Standard Test Conditions are as follows:

  • Cell temperature of 25°C
  • Solar irradiance of 1000 W/m2
  • Mass of air of 1.5

Temperature variation is very important when considering Solar Panels because it will cause the Voc and Isc to vary which can affect sizing considerations.

For example, a solar panel installtion in Scotland where the temperatures can often dip below 0°C will be operating in much different conditions than standard.  You need confidence that the key characteristics aren’t going to stray outside the acceptable range for the inverter, wiring and isolators being used.

Most manufacturers provide temperature coefficients on the datasheet to highlight the variation of these characteristics in different climates. This allows you to more accurately calculate the expected Voc and Isc value at any given temperature.

NOCT - Normal Operating Cell Temperature

Some manufacturers also state figures for the ‘normal operating cell temperature’ or NOCT. This gives a more realistic idea of what characteristics the panel would be expected to have in. These conditions are usually:

  • Irradiance on cell surface = 800 W/m2
  • Air Temperature = 20°C
  • Wind Velocity = 1 m/s
  • Mounting = open back side.

This accounts for the fact that when in direct sunlight the cells will almost certainly be above 25°C. Therefore, NOCT defines the surrounding conditions and then looks at how hot the cells in each panel get. This is useful for comparing panels as it is often more useful to understand a panel will perform compared to another at a specific site. How it performs at 25°C can be a bit too broad for system design given the range of weather conditions we experience up and down the UK.

NOCT values are usually lower and so are not used for sizing in the same way that the STC values are, because again if your array is ‘safe’ at STC then it is almost certainly going to be fine at NOCT conditions, too.

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