• Few VDAs are powered off during the off-peak hours
• Requirement is to keep all the machines to be powered On all the time but reboot the machines after a user logs off

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Run the below PowerShell cmdlets on an elevated PowerShell on a DDC

To set all the VDAs to be powered on,

1. asnp Citrix* 
2. Set-BrokerDesktopGroup "Desktop Group Name" -PeakBufferSizePercent 100

To restart the VDAs after the user logs off, 
Set-BrokerDesktopGroup "Desktop Group Name" -ShutDownDesktopsAfterUse $True

To restart VDAs during peak hours, so that they’re ready for users after they log off, enter:
Set-BrokerDesktopGroup "Desktop Group Name" -AutomaticPowerOnForAssignedDuringPeak $True

https://docs.citrix.com/en-us/citrix-virtual-apps-desktops/install-configure/delivery-groups-manage.html#sessions