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P(in) is taken as the product of the irradiance of the incident light, measured in W/m2 or in suns (1000 W/m^2), with the surface area of the solar cell [m^2].
Here’s a good video explaining solar optimizers: Note - optimizers are not always MPPTs, but the algorithm could be implemented on the optimizers. https://www.youtube.com/watch?v=E9NVLB_OYSU