def _pwr_optfcn(df, loc):
I = _lambertw_i_from_v(df['<STR_LIT>'], df['<STR_LIT>'],           <EOL>df['<STR_LIT>'], df[loc], df['<STR_LIT>'], df['<STR_LIT>'])<EOL>return I * df[loc]<EOL>
Function to find power from ``i_from_v``.