Voltage isn't power. Consider that you have a fixed amount of energy stored in your 12V battery. If you use the system to crank the starter, it will draw lots of power and drain the battery quickly. If it's just sitting with the ignition off, the 12V system still runs the clock chip in the stereo, but that draws an almost negligible amount of power, so the battery will stay up for months, maybe years.
Wattage is a measure of power. Voltage is a measure of electrical "pressure". Current is a measure of electron flow.
That's greatly simplified, but accurate.
So asking "how much power can the socket supply?" would mean that the expected answer would be in a measure of POWER. This is normally wattage (V x I), but could also be horsepower (1HP = 745W, so the socket can supply 0.16 HP) or any other measurement of power.
Over time, we're starting to talke about Watt-Hours, or more normally, Kilowatt-Hours.