Edith Said:

Watts? Volts? Solar panel that can replace a 9v battery?

Watt = Current x Voltage; or, P = I x E
E = P / I
I = P / E

If you want to replace a 9 volt battery with a solar panel, you'll need to make sure that the solar panel can deliver between 100 ma and 200 ma of current under average lighting conditions at a voltage output of 9 volts.

In otherwords, in average light, the solar panel should be able to deliver 200 ma of current into a 45 ohm load resistor while maintaining 9 volts across the resistor.

Remember that a solar cell will deliver something like 0.5 volts at around 20 ma of current. You may need to wire up 18 solar cells in series to get 9.0 volts. You will need 10 groups of these same series of solar cells connected in parallel in order to go from 20 ma of output current to the required 200 ma.

The wiring of individual solar cells in series and parallel is kind of like making a matrix.

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