Powering stuff via the 12v socket affect fuel consumption?

[aclivity]

It may be because it's late, but I can't get my head round this at the moment.

There is no such thing as free energy, so it must put some load on the engine somehow, but the engine is turning the alternator anyway so is powering something from the 12v socket just using energy that was otherwise lost?

Reason I ask is that I am getting stuff ready for the family holiday in a couple of weeks time, and I got the in-car fridge down from the loft to see if it still worked. It works by powering a fan that pulls air across a heat exchanger in the lid of the fridge, running a small motor all the time surely must take some power, and as above, surely it can't be "free"? similarly running the sat nav, or the phone charger (or even for that matter, running the built in DVD player or radio unit) must affect fuel consumption - is it just that it is negligible?

[Jakg]

Yes it takes energy, the alternator is always spinning but as more power is required there is more drag on the engine to make that power, thus more power required and more fuel used.

HOWEVER considering that most 12v sockets are fused ~10A it's really very little power in the grand scheme of things. It uses about the same power as your headlights (flat out).

[S1_RS]

Most people dont realise that the alternator does take power to generate electricity as when they are off the car they can be spun be hand. Once they are connected to the car however they take some force to turn. The more that's asked of them in respect of charging the battery, powering HFS, HRS, stereo etc the more power they require to turn. One of my old Mini's had four 110 watt spotlights with 100 watt main beam's. When I switched the spots on when the car was flat out, it knocked about4-5mph off the car.