If the battery percentage on a brand-new iPhone seems to be disappearing much faster than expected, the first day is a particularly bad time to decide how good or bad its battery life really is.
A freshly configured iPhone can spend hours completing work that normally is not happening every day. Restoring an iCloud backup, downloading apps, syncing an iCloud Photos library, indexing files and messages, and completing system setup all consume power in the background.
At the same time, a new phone usually gets much heavier screen-on use than normal. Camera testing, setting up apps, signing into accounts and exploring new features can turn the first few hours into an unusually demanding workload.
Your battery has not necessarily developed a problem
Apple’s setup process can restore data from iCloud in the background, allowing the iPhone to be used before every app and piece of content has finished downloading. Apple notes in its Quick Start guidance that apps and data continue downloading when an iCloud transfer is chosen.
This is also why a new iPhone can feel warmer than expected. Increased processing and network activity use additional energy, and charging while those tasks are happening can make the temperature more noticeable.
Give the phone time on Wi-Fi and power, particularly overnight. Once synchronization and indexing settle down and your own usage returns to normal, the Battery page in Settings becomes much more useful for judging real-world performance.
Check what is actually using the battery
Settings, Battery shows app and system activity over time. If one app is responsible for an unusually large proportion of background usage after the first few days, that is more useful evidence than the battery percentage during setup.
Battery Health can also provide context, although a genuinely new iPhone should begin with a healthy battery. Features such as Optimized Battery Charging, charge limits and Low Power Mode can be adjusted depending on how you use and charge the device.
Apple says the iPhone 18 Pro generation delivers major battery-life improvements, with eSIM-only iPhone 18 Pro Max models rated for up to 45 hours of video playback. A busy setup day will not resemble that controlled playback test, so launch-day battery drain should not automatically be taken as evidence that something is wrong.