Apple’s Clean Energy Charging feature, released in iOS 16.1, only for the US, is designed to lessen the impact of iPhone users on the environment by reducing the amount of energy consumed while charging the iPhone. In short, reducing carbon footprint while charging.
Basically, when an iPhone is plugged into a charger, it gets a forecast of the carbon emissions from the local energy grid. Then, iPhone will use this information to only charge your iPhone when it can get energy from a cleaner source which leads to slow charging and is turned ‘ON’ by Default on iOS 16.3.1.
Clean Energy Charging works together with Optimized Battery Charging to learn your charging habits in places. But, if your charging habits change or you’re in a new place, the feature won’t work. To disable it, Go to Settings -> battery -> battery health and charging -> clean energy charging and then select “off.”