With your negative Rs the argument to W is negative. If it is too negative W will not evaluate. Clearly root is getting a V value that is too large during its search.
But it works for me. Are you sure you are working with the same sheet that you posted? Download and run the posted sheet to see what you get.
Problems with root generating out of range values are often due to bad guess values (the guess values do matter). Looking at the graphs of the power and the derivative of the power too low a guess value will send root off to a very large value, in this case out of range of the function. The guess value in the posted sheet is .45, rather too small. Inspection shows that the maximum is somewhere in the neighborhood of .5. And the flat slope at low V suggests that if you are going to be off, it is better to be too high than too low. But .45 does work for me. .4 is much too small, and fails with the argument to W out of range.
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� � � � Tom Gutman