This is a more theoretical question other than simulation question, isn't it?
The actual solar power depends on the location and the time of the day as well as other factors. The power spectra is from the "AM 1.5 Standard Spectrum". as for how much the actual power is, I believe Psolar=100mW/m^2 is reasonable but just for illustration purpose. Please find some references to get the exact power for your design.
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