If I understand correctly the only way to get the 20 Watts of power is to use the DC 8mm output.
The USB output only produces 6.5 Watts (5 Volts x 1.30 Amps = 6.5 Watts)?
If we refer to the manufacturer's datasheet, it seems that 20W is potentially achievable only via the 8mm port. As for the USB port, it would seem to be 10.5W max.