Solar panel specifications: Standard Test Conditions (STC), Normal Operating Cell Temperature (NOCT), Open Circuit Voltage (Voc), Short Circuit Current (Isc), Maximum Power Point Voltage (Vmpp), Maximum Power Point Current (Impp), Nominal Voltage
Understanding solar panel specifications and how it affects the choice of solar panels used during installations
There are several terms associated with solar panels and ratings. Go to the back of the solar panel and look at the nameplate or data sheet to get the correct solar panel specification. Below is the explanation of the specification you will find there:
Standard Test Conditions (STC)
STC is the set of criteria to be tested on a solar panel. Since voltage and current changes are based on temperature and light intensity, all solar panels are tested under the same standard test conditions, among other criteria. This includes 25 ° C (77 ° F) cell temperature, 1000 watt light intensity per square meter, which is basically the sun at noon, and 1.5 atmospheric density, or the angle of the sun directly perpendicular to the solar panel at 500 feet above sea level.
Normal Operating Cell Temperature (NOCT)
On top of the roof, the solar panel cells are not 25oC (77℉). They heat up much hotter than that in the sun.
NOCT takes a more realistic view of the real conditions of the real world and gives you power ratings that your solar system is likely to see. It uses 800 watts per square meter instead of 1000 watts, which is closer to a mostly sunny day with dispersed clouds. It uses 20 ° C (68 ° F) air temperature, not solar cell temperature, and includes a 2.24MPH wind cooling the back of a solar panel mounted on the ground (more common in larger solar fields than a residential array mounted on the roof). These ratings are going to be lower, but more realistic than STC.
Open Circuit Voltage (Voc)
The voltage of the open circuit is how many volts the outputs of the solar panel are without load. If you only measure the positive and negative terminals with a voltmeter, you'll read Voc. Since there is no connection between the solar panel and anything, there is no load on it and no current is produced.
This is a very important number as it is the maximum voltage that can be produced under standard test conditions by the solar panel, so this is the number to be used when determining how many solar panels you can wire into your inverter or charge controller in series.
Voc may be produced briefly in the morning when the sun rises first and the panels are at their coolest, but the connected electronics have not yet woken up in sleep mode.
Remember to protect wires from over-current, not over-voltage, fuses and breakers. So, you're going to damage them if you put too much voltage into most electronics.
Short Circuit Current (Isc)
Short Circuit Current is how many amps (i.e. current) the solar panels produce when they are not connected to a load but when the panel wires ' positive and negative terminals are connected directly to each other. If you only measure the positive and negative terminals with an ammeter, you'll read Isc. This is the highest current under standard test conditions that the solar panels will produce.
The Isc is used to determine how many amps can handle a connected device, such as a solar charge controller or inverter.
Maximum Power Point (Pmax)
The Pmax is the sweet spot of the output of the solar panel, located in the graph above at the "knee" of the curves. The combination of volts and amps leads to the highest wattage (Volts x Amps = Watts).
When using a Maximum Power Point Tracking (MPPT) charging controller or inverter, this is the point that the MPPT electronics attempts to keep the volts and amps at maximizing power output. The wattage of a solar panel as the Pmax where Pmax = Vmpp x Impp is listed
Maximum Power Point Voltage (Vmpp)
The Vmpp is the voltage at the greatest power output. It is the actual voltage you want to see when connecting under standard test conditions to the MPPT solar equipment (like a MPPT solar charge controller).
Maximum Power Point Current (Impp)
If the power output is the largest, the Impp is the current (amps). It is the actual amperage you want to see when connected under standard test conditions to the MPPT solar equipment.
Nominal voltage is the one that makes many people confused. It's not a real voltage you're going to measure. Nominal voltage is a category.
For instance, a nominal 12V solar panel has approximately 22V Voc and approximately 17V Vmp. A 12V battery (which is actually about 14V) is charged.
A solar panel with 12V charging controller, 12V battery bank and 12V inverter is used. By cabling two 12V solar panels together in series, you can create a 24V solar array.
When you move away from battery-based solar systems, it begins to get tricky and the increments of 12V are no longer needed. They have a voltage too high to charge a 12V battery bank with a traditional charging controller, but a voltage too low to charge a 24V battery bank. MPPT charging controllers can change the voltage output to enable a battery system to use them.
Note: Each of the various specifications stated above must be well understood to get the best out of a solar panel. At Wavetra Energy Academy, we take time to analyze each of the specifications of the solar panel and how they play out in your installations. Register today and become a solar energy expert.
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