Get the monthly average folder size

Get-ArchiveSize.ps1

<#
Given the list of folders and the dates over which to check then this script
will group the files by month for each folder between the given dates and get
the sum of the file sizes. So you will get a calculated property of 'folder',
'month', 'size' (where file size is in MB to two decimal places). Then that
calculated property is grouped by 'folder' and the cumulative file size over the
selected months is averaged. So you will have a calculated property of 'folder',
'monthly' (where 'monthly' is a monthly average in MB).
#>
$archives = @{
  FileLocation     = '\\server\archive'
  ArchiveLocation  = '\\another\archive'
}# Uses the last three complete months.
$start = Get-Date -Year 2020 -Month 12 -Day 01
$end = Get-Date -Year 2021 -Month 02 -Day 28
$archives.Values `
| ForEach-Object { `
    $folder = $_;
  Get-ChildItem $_ `
  | Where-Object { !$_.PSIsContainer } `
  | Where-Object { $_.LastWriteTime -le $end -and $_.LastWriteTime -ge $start } `
  | Group-Object { $_.LastWriteTime.Date.Month } `
  | Select-Object `
  @{ n = 'Size'; e = { "{0:N2}" -f (($_.Group | Measure-Object Length -Sum).Sum / 1mb) } }, `
  @{ n = 'Month'; e = { $_ | Select-Object -Expand Name } }, `
  @{ n = 'Folder'; e = { $folder } } } `
| Group-Object Folder `
| Select-Object `
@{ n = 'Monthly'; e = { "{0:N2}" -f (($_.Group | Measure-Object Size -Average).Average) } }, `
@{ n = 'Folder'; e = { $_.Group | Select-Object -Expand Folder -First 1 } }