What happens if power gets interrupted during IOS 15 update?

It will depend on:

• Whether you are updating (within the same major iOS version) or upgrading (between major iOS versions).
• Where in the update/upgrade process that your phone loses Internet connectivity (regardless of the reason.)

In the downloading phase, the result most likely, is that you will have an incomplete iOS file on your device. This can easily be deleted, and the update/upgrade process can be re-initialized.

During the installation phase, the result may be a corrupted iOS which may result in the phone not being able to be used. This would require a complete restore ... and where current backups will be critical if you want to restore its data.

The Update may also be interrupted if the device does not have enough battery power to complete the update and that can corrupt the iPad. This will cause all data on it to be lost, and will require the use of a computer to restore to working order.

If the device has enough battery power to continue with the update, then nothing should happen if its disconnected from power.

Nothing happens. iPhone has battery power. Keep it over 50%.