title: Pandas dataframe first instance of value in column
body: <p>I have df:</p>
  
  <pre><code>                     Voltage
  01-02-2017 00:00       13.1
  01-02-2017 00:01       13.2
  01-02-2017 00:02       13.3
  01-02-2017 00:03       14.1
  01-02-2017 00:04       14.3
  01-02-2017 00:04       13.5
  </code></pre>
  
  <p>I would like the time (hh:mm) of the first instance of when the value in the Voltage column >=14.0. There should only be one time value in column 'Time of Full Charge'.</p>
  
  <pre><code>                     Voltage   Time of Full Charge
  01-02-2017 00:00       13.1
  01-02-2017 00:01       13.2
  01-02-2017 00:02       13.3
  01-02-2017 00:03       14.1         00:03
  01-02-2017 00:04       14.3
  01-02-2017 00:04       13.5
  </code></pre>
  
  <p>I am trying something along these lines, but cannot figure it out:</p>
  
  <pre><code>df.index = pd.to_datetime(df.index)
  df.['Time of Full Charge'] = np.where(df.['Voltage'] &gt;= 14.0), (df.index.hour:df.index.minute))    
  </code></pre>