How to read the solar panel technical specifications?
There are several terminologies that are associated with a solar panel’s data sheet. It could be quite confusing if you don’t understand what these mean when you read a specification sheet. We are going to explain each of them to help clarify these terms and ratings.
Standard Test Conditions (STC)
STC is the set of criteria that a solar panel is tested at. The voltage and current vary with the changes of temperature and intensity of light, so all solar panels shall be tested to the same standard test conditions. This includes the photovoltaic cells’ temperature of 25℃, light intensity of 1000 watts per square meter, which is about the same as the sun at noon, and the atmospheric density of 1.5, or the sun’s angle directly perpendicular to the solar panel at 152 meter above the sea level.
Normal Operating Cell Temperature (NOCT)
NOCT takes a more realistic view of actual real world conditions, and gives you power ratings that you will likely actually see from your solar system. Instead of 1000 watts per square meter, it uses 800 watts per square meter, which is closer to a mostly sunny day with some scattered clouds. It uses an ambient temperature of 20℃ (68℉), not a solar cell temperature, and includes a 2.24MPH wind cooling the back of a ground mounted solar panel (more common in larger solar fields than a roof mounted residential array). These ratings will be lower than STC, but more realistic.
Rated Output Specifications and Solar Panels
Rated output for solar panels at different light intensities (W/m2). The “knee” of the curves is where the most power is produced, and the voltage & current is optimized.
Open Circuit Voltage (Voc)
Open circuit voltage is amounts of voltage that the solar panel outputs with no load on it. If you just measure with a voltmeter across the positive and negative leads, you will get Voc reading. Since the solar panel isn’t connected to anything, there is no load on it, and it is producing no current.
This is a very important number, as it is the maximum voltage that the solar panel can produce under standard test conditions, so this is the number to use when determining how many solar panels you can wire in series going into your inverter or charge controller.
Voc will potentially be briefly produced in the morning when the sun first comes up and the panels are at their coolest, but the connected electronics haven’t woken up out of sleep mode yet.
Remember, fuses and breakers protect wires against over-current, not over-voltage. So, if you put too much voltage into most electronics, you will damage them.
Short Circuit Current (Isc)
Short Circuit Current is the amounts of amps (current) the solar panels are producing when not connected to a load but when the plus and minus of the panels wires are directly connected to each other. If you just measure with an amperage meter across the positive and negative leads, you will get Isc readings. This is the highest current the solar panels will produce under standard test conditions.
When determining how many amps a connected device can handle, like a solar charge controller or inverter, the Isc is used, generally multiplied by 1.25 for National Electrical Code (NEC) requirements.
Maximum Power Point (Pmax)
The Pmax is the sweet spot of the solar panel power output, located at the “knee” of the curves in the graph above. It is where the combination of the volts and amps results in the highest wattage (Volts x Amps = Watts).
When you use a Maximum Power Point Tracking MPPT) charge controller or inverter, this is the point that the MPPT electronics tries to keep the volts and amps at to maximize the power output. The wattage that a solar panel is listed as is the Pmax where Pmax = Vmpp x Impp (see below).
Maximum Power Point Voltage (Vmpp)
The Vmpp is the voltage when the power output is the greatest. It is the actual voltage you want to see when it is connected to the MPPT solar equipment (like an MPPT solar charge controller or a grid-tie inverter) under standard test conditions.
Maximum Power Point Current (Impp)
The Impp is the current (amps) when the power output is the greatest. It is the actual amperage you want to see when it is connected to the MPPT solar equipment under standard test conditions.
Example of SolarWorld SunModule solar panels Standard Test Conditions (STC) and Normal Operating Cell Temperature (NOCT) ratings.
Nominal voltage is the one that confuses a lot of people. It’s not a real voltage that you will actually measure. Nominal voltage is a category.
For example, a nominal 12V solar panel has a Voc of about 22V and a Vmp of about 17V. It is used to charge a 12V battery (which is actually around 14V).
Nominal voltages let people know what equipment goes together.
A 12V solar panel is used with a 12V charge controller, a 12V battery bank, and a 12V inverter. You can make a 24V solar array by wiring two 12V solar panels together in series.
12V solar panels charging a 12V battery with a traditional 12V PWM charge controller.
It starts to get tricky when you move away from battery based solar systems, and the 12V increments are no longer necessary. Grid tie solar panels with 60 cells are often referred to as 20V nominal panels. They have too high of a voltage to charge a 12V battery bank with a traditional charge controller, but too low of a voltage to charge a 24V battery bank. MPPT charge controllers can change the voltage output to allow them to be used in a battery system.
A 20V nominal solar panel goes through an MPPT solar charge controller so it can efficiently charge a 12V battery.
Number of cells
Open Circuit Voltage (Voc)
Max Power Volts (Vmp)
Above: Approximate voltages to determine nominal voltage of solar panels.