How to open excel workbook from powershell for automation

You need to open it as a ComObject.

$Excel = New-Object -ComObject Excel.Application
$Workbook = $Excel.Workbooks.Open($FilePath)

In that example you would have needed to define $FilePath as the full path to the Excel file that you are trying to open.

I've found a nice snippet which also runs a macro here

# start Excel
$excel = New-Object -comobject Excel.Application

#open file
$FilePath = 'C:\temp\Book1.xlsm'
$workbook = $excel.Workbooks.Open($FilePath)

#make it visible (just to check what is happening)
$excel.Visible = $true

#access the Application object and run a macro
$app = $excel.Application
$app.Run("Macro1")