Example for stack overflow.

<!DOCTYPE html>
<html>

  <head>
    <link rel="stylesheet" href="style.css">
    <script src="script.js"></script>
  </head>

  <body>
    <p id="results"></p>
  </body>

</html>

var
  genId = 2,
  curStatus = 'off',
  totalCurrentPower = 62,
  powerProduced = 62
  results = document.getElementById('results');

function changePowerTotal(totalCurrentPower, generatorId, currentStatus, powerProduced) {
  if (currentStatus === 'off') {
    var offPowerTotal = totalCurrentPower - powerProduced;
    alert('Generator # ' + generatorId + ' is now ' + currentStatus + ' , removing ' + powerProduced + 'mw, for a total of ' + offPowerTotal);
    return offPowerTotal;
  }
  else {
    var onPowerTotal = totalCurrentPower - powerProduced;
    alert('Generator # ' + generatorId + ' is now ' + currentStatus + ' , removing ' + powerProduced + 'mw, for a total of ' + onPowerTotal);
    return onPower
  }
}

changePowerTotal(totalCurrentPower, genId, curStatus, powerProduced);

/* Styles go here */