I want to use a solar cell to find the intensity of solar radiation by using the input of the solar cell on the Arduino board. What's the power rating of a UNO Board that I can consider as the maximum limit for the power needed from the solar cell.
The answer is "depends."
Some information can be found here: http://arduino.cc/en/Main/ArduinoBoardUno
However, you need to calculate or measure the amount of current the board is drawing in your application to determine actual power usage.
I think you mean the "current draw" instead of the power rating. Just the bare Arduino without any shields etc. the current draw is in the range 30-40mA in my experience.
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