Solar 101


Standard test conditions (STC)

To enable comparisons between different panels, the performance of all panels are specified against a set of conditions used industry-wide called Standard Test Conditions (Ex: cell temp of 25 degrees Celsius and an irradiance of 1000W/m2 with an air mass 1.5). These conditions are extremely unlikely to occur in real life.

Open-circuit voltage (Voc)

The open-circuit voltage is the maximum voltage that the solar panel can produce with no load on it. (Ex: measured with a multimeter across the open ends of the wires attached to the panel). If two or more panels are wired in series it will be the Voc of panel 1 + of panel 2, etc… The voltage is generally highest mid-morning as the sun rises rapidly and the panel temperature is still quite low.

The Voc + aprox 3.5% must be less than the maximum solar voltage permitted by the solar charger controller. Some controllers shut down if exceeded, while some may continue to operate but the lifespan of the controller could be compromised.

Short-circuit current (Isc)

Short-circuit current is the current that flows out of the panel when the positive and negative leads are shorted together. The current can be measured by passing the current through a multimeter configured to measure amps (This does not harm the panel, but care must be taken to avoid arcing!).

The Isc + 20% is recommended to determine the required current handling capacity of a compatible solar-charge controller. This is the highest current the solar panels will produce under standard test conditions – note that under a clear sky, at midday in summer, and tilting the panel towards the sun you would get significantly more current. SunTech solar has seen more than 7.0A from SUNPOWER e-flex 110W panels.

Voltage at Maximum power (Vmp)

The voltage at maximum power is the voltage when the power output is at it’s greatest. It is the actual voltage you want to see when it is connected to the MPPT controller under standard test conditions. In practice the actual Vmp will vary during the course of the day and with temperature, shading, soiling of the panel, etc. You can measure this voltage with a multimeter at the solar input terminals of an MPPT during the bulk-charge mode. This information is an important factor in deciding the proper MPPT controller.

Current at maximum power (Imp)

The Imp 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 an MPPT controller under standard test conditions in bulk-charge mode. The actual current varies with sun strength on the panel. This information is an important factor in deciding the proper MPPT controller.

Maximum power point (Pmax)

The Pmax is the sweet spot of the solar panel power output, where the combination of the volts and amps result in the highest wattage (Volts X Amps = Watts).

The “brains” inside an MPPT controller periodically measures the panel voltage under varying loads and then adjust the solar input circuit to balance the volts and amps to maximize the power output during bulk-charge mode. The wattage that a solar panel is listed as is the Pmax = Vmp X Imp at standard test conditions.


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