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The lines on the graph represent the Classic output current at different Max Power Points Voltage (not VOC). To estimate the Max Power Point of your system; first you have to find out the VOC of the Photovoltaic strings connected to the Classic. For example If 5 Modules in series in one string and STC VOC rating is 36.6V, then the VOC of the string equals: 5(modules) * 36.6v(STC VOC) = 183V.

The rule of thumb says that the MPP will be around 80% of the string's VOC; 183v * 0.8 = 146.4v On the graph of the Classic model you have, the X axis represents the Battery Voltage and the Y axis is the Output current. Knowing that, go to the corresponding graph and look for the a line close to your system's MPP voltage and you can then determine the max output power for your system design.