Ratbag,
I only carry one 1kg in each of my cars. I figure it is more to save the occupants than the car. Leave the car for the insurance to deal with. As you say, we are driving cars, not Fire Trucks.
A 1kg dry chemical (dry powder) will cope with things like electrics, and most under bonnet fires. Once a fire has gotten into fuel lines, or the interior, conosider it a loss.
Make sure when you buy an extinguisher that you get one that has the letters A,B,E in it. These are more versitile than just A,B types.
A=class A, papers, wood, rags, etc.
B=class B, Fuels, oils, etc (Liquids)
E=electrical, pretty obvious.
If you were travelling more remote, make sure each vehicle has an extinguisher, or if you are travelling alone, then having two would not be a bad idea.
Beigewagon.