Charging iPhone 14 plus via 5W adapter

Your iPhone 14 is designed to use a 20W adapter and will not be harmed by it. The charging control circuits in an iPhone are intelligent and will only take as much power as the phone needs, up to the limit of the charging adapter's output rating. I regularly use the 30W and 2-port 35W USB-C adapters for all my devices with no ill effects.

I have one minor concern about the 5W adaptor. The rating is so low it might struggle to charge the battery in a reasonable time. However this will not damage your iPhone, even if the phone is not fully charged when you need it.

Does charging iPhone 14 plus via 5Watt adapter rather than 20Watt adapter is better for battery life?

No

(I don’t mean using 5W adapter always, I mean sometime like twice a week in the night).

Same answer. The 5w adapter is not better for battery life.

A fact that most users do not know......the "charger" is in the phone, not in the adapter.

No change 5w adapter 5w adapter is good working your phone and battery life save long time