Charging a mobile for 1 hour uses 2 watts.
Leaving the mobile connected to the charger after the battery is full for 7 hours uses 14 watts.
Leaving the charger plugged in and turned on, but not connected to the mobile for 16 hours uses 24 watts.
Over 1 year you use 14.6KWh.
Turning the charger off when you’re done is equivalent to watching a 32” LCDTV (170W) for 1 hour per day for 11 weeks, or boiling a kettle over 100 times (1500W kettle taking 5 minutes to boil).
(I’m not recommending the product, mind you – I suspect there’s more embodied energy in there than you’d save. Just turn off the plug.)