So, in 24hrs my factory reset phone with no apps or data installed goes from 100% to under 40%. That seems excessive considering it had no use in that time.
I suppose it could be coincidence that the point I went from IOS13 to IOS14.2 suddenly the battery started to fail..... ...but thats pretty unlikely.
Settings > Battery
That will tell you what has been draining the battery, along with Screen On/Off time etc.
Settings > Battery > Battery Health
That will tell you if the battery is Donald or not.
ETA, Battery consumption is fine for me with iOS14 (iPhone11Pro)
Batt health is running peak, 91% max capacity.
Application using most is 'dangle berries'.
Could be a failing battery, after all, this one is coming up to 2 years old, though the cynic in me thinks Apple fitted crap batteries when they were forced to do it cheap. 6S looks a bit more of a pain to change battery than previous. Debating if I can be arsed, or get another £200 Galaxy.