Why does my iPhone lose battery even when not in use?

Battery Performance Tips:

• iPhone Battery and Performance.
• Maximizing Battery Life.
• About battery usage.
• Check the iPhone battery health and usage.
• Optimized Battery Charging.
• Use Low Power Mode.

If your battery health reaches 80% before you reach one year from the date of purchase, then you qualify for a free battery replacement. 🔋😁

Need more help?

• Chat now or call 1-800-APLCARE.
• International Support.

Click ▶︎ iPhone User Guide. 📖

Good luck! 👋🏼😉

Unless your phone is turned completely off it still performs certain services when in sleep mode. Even when turned off, things like the clock are still using power (not much though).

Check the Settings > Battery feature to see if there are apps or services running that use a lot of power. If there are, decide if you need them running all the time, can turn off a service when an app is in the background, or disable the feature completely. Read this support article to learn about the battery health feature. —> iPhone Battery and Performance - Apple Support

Refer to this post for good tips on reducing battery use on your device --> https://discussions.apple.com/thread/255063652?answerId=259418848022#259418848022

Addendum to #11. Use 4G/LTE instead of 5G. Use 5G with your iPad - Apple Support

Here is a related post in the context of device heating. --> iphone pho max 14 phone gets hot - Apple Community