I'm running a v2 instance and from the documentation aws states you should only be paying for resources that you are actually using. I have an instance than is most of the time at 0 connections but it never scales down under 2 ACUs. See images below for reference. I have the instance setup to scale between 0.5-16ACU. But it doesn't seem to matter the load it always stays at a baseline of 2ACUs.
