[RedDevil]

Originally Posted by 2000mc

So for every gallon of gas a car tries to turn into electricity, it puts out around 3kwh? So you replace buying a gallon of gas with 35 - 40 cents from your outlet. Sounds like it works out pretty well then.
Or saying you need 12kwh? Or more like $1.50/gal break even?

I meant the latter. About $1.50 cost of electricity for saving a gallon of gas.

At traditional gasoline engine is about 25% effective in turning gas into mechanical power, max.
Now most of the time it is on partial load, that's less than 25% effective, but I assume the extra load of running the alternator makes the engine work slightly harder, so slightly more efficient; the difference between both may well work out to 25%, a tad over 8 kWh.

The 75% belt and alternator efficiency for turning motion to electricity is a bit on the high side maybe, just to be sure. -> 6 kWh
50% efficiency on charging the battery from the mains is probably way low, again to prevent pro bias on the alternator delete side.
You need 12 kWh from the mains then to get those same 6 kWh of battery power.

The fact stands that charging the battery from the mains rather than the alternator is much cheaper.
Imagine how it works out with our ridiculously high European fuel prices

The OP may have been trolling, never mind we'll turn it into an exercise of getting the math right.
There are lots of reasons why it can be impractical to do an alternator delete, but energy efficiency is not one of them.