Yes that lead appears to be trying to get blood put of a stone. USB sockets give nominally 5 volts in the 1 to 3 amp range. Give the loss in the voltage convertor you would be very lucky to get even the 8 watts mentioned.
If you don't have a cigar lighter socket and need a 12 volt up to 15 amp supply fit a Hella style socket and wire it in to a 15 -20 amp fused battery supply. Cigar lighter sockets are rubbish.
