How is power generation computed in a circuit?
Power = voltage x current so, decide what voltage you are interested in i.e.


Voltage across the LED
Voltage across the resistor and LED (solar panel output)


... and multiply instantaneous values of voltage with current (presumably the current mentioned in your question).

That gives you instantaneous watts and you can average this to give a decent picture of average power (what you would be billed on). If you want to calculate watt hours then you need to integrate the watts over time.