Getting montly cpu contention value?

Hi guys , i am fairly new with powercli , i am trying to get montly value of cpu contention, by my understanding  with this script  (get the script from this forum) i get the average of today (maybe i am wrong):

$Cluster = 'clustername'

$stat = 'cpu.latency.average'

$CPU_contention = (Get-Stat -Entity $esx -Stat $stat -Realtime -MaxSamples 1 | Measure-Object -Property Value -Average).Average

$CPU_contention='{0:N1}' -f $CPU_contention -replace ",","."

$CPU_contention

i think i must replace  -Realtime for -Interval 7200 to get Historical Interval 3  , but i get the error that the hosts of the cluster doesen't have the cited statistics.

any hint , will be welcome

                                        Martin