How to convert .xls to .csv using Powershell without Excel installed

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Depending on what you already have installed on your system you might need the Microsoft Access Database Engine 2010 Redistributable for this solution to work. That will give you access to the provider: "Microsoft.ACE.OLEDB.12.0"

Disclaimer: Not super impressed with the result and someone with more background could make this answer better but here it goes.

Code

$strFileName = "C:\temp\Book1.xls"
$strSheetName = 'Sheet1$'
$strProvider = "Provider=Microsoft.ACE.OLEDB.12.0"
$strDataSource = "Data Source = $strFileName"
$strExtend = "Extended Properties='Excel 8.0;HDR=Yes;IMEX=1';"
$strQuery = "Select * from [$strSheetName]"

$objConn = New-Object System.Data.OleDb.OleDbConnection("$strProvider;$strDataSource;$strExtend")
$sqlCommand = New-Object System.Data.OleDb.OleDbCommand($strQuery)
$sqlCommand.Connection = $objConn
$objConn.open()

$da = New-Object system.Data.OleDb.OleDbDataAdapter($sqlCommand)
$dt = New-Object system.Data.datatable
[void]$da.fill($dt)

$dataReader.close()
$objConn.close()

$dt 

Create an ODBC connection to the excel file $strFileName. You need to know your sheet name and populate $strSheetName which helps build $strQuery. When then use several objects to create a connection and extract the data from the sheet as a System.Data.DataTable. In my test file, with one populated sheet, I had two columns of data. After running the code the output of $dt is:

letter number
------ ------
a           2
d          34
b           0
e           4

You could then take that table and then ExportTo-CSV

$dt | Export-Csv c:\temp\data.csv -NoTypeInformation

This was built based on information gathered from:

1. Scripting Guy
2. PowerShell Code Repository