If a user is aware a choice is being made to reduce carbon emissions, then the application becomes a carbon-aware application and not just a carbon-efficient application.
Does a 100% supply of renewable energy make computing guilt-free? The answer depends on if you are being powered-by or matched-by renewables and is always "no".
Alongside the 8 principles of Sustainable Software Engineering, there are 2 philosophies. For many of us, sustainability is more than a task. It's a purpose and a movement. Where does Sustainable Software Engineering sit in that broader sustainability movement?