How many watts does a mobile phone need?

From wall socket:
To charge up the phone it needs 2-5 Watts (including charger dissipation).

From battery:
In extreme conditions, it could drain 5 Watts (ending up a fully charged good battery within 1 hour)

During a phone conversation it could use up to 800 mW.

In Standby mode 5-30 mW (for me: standby means no processor load, no lights, no GPS, no Bluetooth, no WIFI, no sensors, ...)