Low power mode.
How much will low power mode extend the battery life of an iPhone or iPad?
iPad, iPadOS 15
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While Low Power mode will reduce battery utilisation for your iPad and iPhone when the feature is enabled, the actual benefit that you see will be heavily influenced by your device settings. As such, an absolute answer is impossible to provide.
With an understanding of how Low Power mode changes specific functions, you’ll be able to better understand how it may help to extend battery autonomy of your devices before they must be recharged.
Use Low Power Mode to save battery life on your iPhone or iPad - Apple Support