I often see the following argument: Marginal electricity is coal, EVs represent a new demand, therefore EVs run on coal.
This is wrong.
As an electricity consumer in Norway, I can choose what power I want to buy. That destroys the whole marginality thing in one crushing blow. For the sake of argument, let's pretend that I don't have a choice.
If I lived in a more sunny country, I might have installed solar PV on my roof. Let's pretend I don't have that choice either.
Marginal power is the power that is least expensive to generate that is not already in use. If demand rises some small increment, marginal power will meet that demand. If it falls, marginal power will be shut down.
It's not quite that simple, however.
Marginality also assumes that no one is able to plan ahead and that everyone immediately finds out what the best course of action is. The world does not work that way.
There will also be different types of marginal power for different scenarios. There are four classes of power plants: 1) Base load, running 24/7 *EDIT deleted "full power" which I don't really know whether is true or not * 2) load-following, which follow the load curve throughout the day, 3) peak power which is only started up when load is approaching the average high and 4) reserve power, plants that are so inefficient that they have been closed down completely and are only started up when base power demand is higher than usual, like during a cold spell in the winter.
If load is very high and peaking plants are already running, then more demand will be met by injecting more gas into the turbines, or by building new gas turbine plants if the load is getting dangerously high. If we're talking about a general increase in night-time demand, the peaking plants are unaffected. This will be met by running base load plants harder or building new base load plants.
If, all of a sudden, huge numbers of new EVs were to be charged during the night, then reserve power would have to be fired up. They were shut down because of high fuel costs, so if the new demand can be expected to last, then it makes sense to build modern plants so they can be shut back down again.
The introduction of EVs will be a slow and predictable process. Reserve power will not be used to meet such a slow increase in base power requirements, this is something that can be planned for. I don't think the marginality concept is very useful in this situation.
Also, if marginal power is coal, then why are new gas turbine plants being built all the time?
Finally, with a smart grid and lower charge cycle cost, EVs can help smooth out the demand curve.