It looks as if you are looking for the active memory in that case.
The following gets the average over 7 days.
Get-VM |
Select Name, MemoryGB,@{N="MemoryConsumedGB";E={
[math]::Round((
(Get-Stat -Entity $_ -Start (Get-Date).AddDays(-7)-Stat "mem.active.average" |
Measure-Object -Property Value -Average | Select -ExpandProperty Average)/1MB),1)}}