Private Declare Function SendMessage Lib "user32" Alias "SendMessageA" _
(ByVal hWnd As Long, ByVal wMsg As Long, _
ByVal wParam As Long, ByVal lParam As Long) As Long

Const WM_SYSCOMMAND As Long = &H112&
Const SC_MONITORPOWER As Long = &HF170&
Const HWND_BROADCAST As Long = &HFFFF&
Const MONITOR_OFF As Long = &H2&
Const MONITOR_ON As Long = -&H1&

Sub PowerOff()
    SendMessage HWND_BROADCAST, WM_SYSCOMMAND, SC_MONITORPOWER, MONITOR_OFF
End Sub