Did you already do some basic troubleshooting, like:
Restart the phone?
Soft reset the phone?
Is the OS and all apps updated to the latest version?
Did the issue first occur after installing one or more apps?
- It only takes one misbehaving app to spoil things.
The generic power saving tips in the FAQ's are outdated in my opinion.
A Nokia 8 on Android Oreo, with everything turned on, mobile data, WiFi, location, Bluetooth, NFC, e-mail sync, Google Play services etc. drain less than 1% per hour in good coverage when the screen is off, and even less when Do Not Disturb is activated during the night.
I've seen idle consumption in poor coverage up to 2% per hour when the phone need more power to sync mail and perform other online background tasks, but more than that suggests something is not right.
A defective or worn-out battery typically recharges to 100% faster than usual and drains fast when the phone is used actively.
Does this help?