Use the menu item Window > Power Inspector to open the Power Inspector window.

You can also open Power Inspector as a new tab using the menu item Window > Power Inspector > New Tab. When you do this Power Inspector will be opened as a new tab next to the window you were last using.

You can also open a new Power Inspector tab using the keyboard shortcut Ctrl+Shift+T (Cmd+Shift+T on macOS).

Tip: it is possible to open multiple instances of the Power Inspector window.