Short answer
Give it 48 hours after setup, update to the latest iOS, turn off Connectivity Assist if you don't need it, and check Settings › Battery for the app responsible.
Fast drain in the first days is almost always software — your new phone is busy setting itself up.
Fixes in order
- Wait 48 hours after setup or a restore. Photo analysis, search indexing and app downloads run in the background.
- Update to iOS 27.0.1 or later and update your apps.
- Turn off Connectivity Assist (Settings › Wi-Fi) if your Wi-Fi is reliable — it keeps cellular data active alongside Wi-Fi.
- Open Settings › Battery and look for apps with high background activity.
- Turn off Background App Refresh for apps that don't need it.
- Lower brightness or let Auto-Brightness manage it.
- If you're often in weak signal, try Wi-Fi Calling and Low Data Mode.
When it's not software
If the phone drains quickly while idle overnight after all of the above, record the Battery screen and contact Apple Support for a diagnostic.
Sources
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