The Windows 8.1 Update released on April 8th 2014 via Windows Update, added a Power button on the Start screen.
On some devices you may not see the Power button. On devices that are not tablets like laptops, desktop PCs and All-in-Ones, the Power button should appear on the Start screen after installing the update. On most tablets you will not see the Power button on the Start screen as they have connected standby and a physical power button that lets you quickly shut down or put the device to sleep. However, on tablets larger than 8.5-inches without connected standby, you will see the Power button on the Start screen.
This tutorial will show you how to add or remove the Power button from the top right corner of your Start screen in Windows 8.1 Update.