With an iOS update to v15, my iPhone started draining battery and getting out of power within 10-12 hours. After several tests I found the root cause - ‘Always on VPN’ via Wireguard (but it reproduces with Tailscale and Cloudflare WARP).
The ‘Screen Off’ is used by Wireguard:
The solution found by accident: iPhone switched on the ‘Low power mode’ automatically when reaching 20% battery and survived on the remaining charge almost the same number of hours as for the first 80%.
If you enable the ‘Low power mode’ - the phone will switch automatically on the next charge.
With ‘Shortcuts’ you can set it up permanently: “How to Keep Low Power Mode Enabled Permanently on Your iPhone”
Result - almost two days on battery back: