This is from my experience bro.
Do not forget that watt=volt*amp (5 volts * 2.0 amp=10 watts).
Which gives us that 10 watts equal to 2 amp home wall charger or 9w which would be equal to 1.8 amp charger if you'll try a 6w charger It's just too low for the nexus it will charge up to 1.2 amp,and this is honestly not enough.
Also the Nexus 5 chargers really slow if the phone being used while charging (which in this occasion could be GPS,Music,Bluetooth),so yeah,9 watt at least.
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