This PowerShell Workflow runbook connects to Azure using an Automation Credential and Starts/Stops a VM/a list of VMs/All VMs in a Subscription in-parallel.

Stop-Start-AzureVM-By-Tag.ps1

Workflow Stop-Start-AzureVM-By-Tag 
{ 
    Param 
    (
        [Parameter(Mandatory=$true)][ValidateSet("Start","Stop")] 
        [String] 
        $Action,
        [Parameter(Mandatory=$true)][ValidateNotNullOrEmpty()] 
        [String] 
        $TagName="ScheduledOnOff", 
        [Parameter(Mandatory=$true)][ValidateNotNullOrEmpty()] 
        [String] 
        $TagValue="Yes"
    ) 

    Write-Output "Logging in to Azure..."
    
    $servicePrincipalConnection = Get-AutomationConnection -Name "AzureRunAsConnection"

    $connectionResult = Connect-AzureRmAccount `
    -ServicePrincipal `
    -TenantId $servicePrincipalConnection.TenantId `
    -ApplicationId $servicePrincipalConnection.ApplicationId `
    -CertificateThumbprint $servicePrincipalConnection.CertificateThumbprint

    Write-Output "Logging in to Azure succeeded"
 
    try 
    {
        $AzureVMNames = (Get-AzureRmVM | ? {$_.Tags[$TagName] -eq $TagValue}).Name
        [System.Collections.ArrayList]$AzureVMs = $AzureVMNames

        Write-Output "Looping all VM names"; 
        foreach -parallel ($AzureVM in $AzureVMs) 
        { 
            Write-Output "($Action) VM ($AzureVM)..."; 
            if($Action -eq "Stop") 
            { 
                Get-AzureRmVM | ? {$_.Name -eq $AzureVM} | Stop-AzureRmVM -Force 
            }
            else 
            { 
                Get-AzureRmVM | ? {$_.Name -eq $AzureVM} | Start-AzureRmVM
            } 
            Write-Output "($Action) VM ($AzureVM) done"; 
        } 
    }
    catch {
        Write-Output "Looping all VM names: Got error ($_.Exception.Message)"
    }

    try 
    {
        $AzureVMssNames = (Get-AzureRmVmss | ? {$_.Tags[$TagName] -eq $TagValue}).Name
        [System.Collections.ArrayList]$AzureVMScaleSets = $AzureVMssNames

        Write-Output "Looping all VM ScaleSet names"; 
        foreach -parallel ($AzureVMScaleSet in $AzureVMScaleSets) 
        { 
            Write-Output "($Action) VM ($AzureVMScaleSet)..."; 
            if($Action -eq "Stop") 
            { 
                Get-AzureRmVmss | ? {$_.Name -eq $AzureVMScaleSet} | Stop-AzureRmVmss -Force 
            }
            else 
            { 
                Get-AzureRmVmss | ? {$_.Name -eq $AzureVMScaleSet} | Start-AzureRmVmss
            } 
            Write-Output "($Action) VM ($AzureVMScaleSet) done"; 
        } 
    }
    catch {
        Write-Output "Looping all VM ScaleSet names: Got error ($_.Exception.Message)"
    }

    Write-Output "Script completed"
}