Here is a script that changes the power plan to Balanced or Power Saver depending on idle time. Just schedule it with Task Scheduler.
if A_TimeIdlePhysical > 1200000
{
# Power Saver
Run C:\Windows\System32\powercfg.exe /setactive a1841308-3541-4fab-bc81-f71556f20b4a
return
}
else
{
# Balanced
Run C:\Windows\System32\powercfg.exe /setactive 381b4222-f694-41f0-9685-ff5bb260df2e
return
}