let's suppose that the load was, on a sunny Sunday morning, 32GW
Let's suppose that the nuclear kettles were boiling out 8GW, the interconnects 3.5GW, Solar 8GW, wind 8GW, and coal 4GW.
All of these figures are currently realistic.
Let's suppose the hydro is ticking on and off, and pumped storage is absorbing slight oversupplies and pricing anomalies. Some combined gas is whirring at near-tickover.
You really can't say "you can rest assured that your "green" car is gas or coal powered." because very little of the energy is coming from gas or coal.
Are you having a problem understanding this ?
Wind is pumping out what it can - and there's no more to be had. Ditto solar. Ditto nuclear. Ditto hydro in that it'll be running at whatever it's water supply can do.
So, when you add some INCREMENTAL
load to the grid, where will the balancing generation come from ? It can't come from wind, solar, nuclear - they are already throwing out what they can. So it'll come from those gas and coal stations that have spare capacity.
So, while you are correct in that the actual watts you get out of the socket will come from a mix of supplies - the INCREMENTAL
load you have added can only be supplied by gas/coal.
As an analogy. There's this big bucket, with a few "generators" throwing balls into it. Windy is throwing green balls in, but he has a limited and variable supply of them. He's been told to throw in all he can get his hands on. Sunny has a similarly limited and variable supply of yellow balls - and again has been told to throw in all he can get hold of. Nuky has a fairly regular supply of white balls - and gets to throw all of them into the bucket.
Lastly, there's Gaz with blue balls, and Chalky with black balls - they've been told that between then they are put in enough to keep the bucket full, and nearly all of the time they've got loads of balls spare.
Meanwhile a variable number of people are coming along and grabbing handfuls of balls. What they get will be a mix of colours - there's no way that they can have just green balls, so these "green" tariffs are nothing more than greenwash.
When you come along to grab a handful of balls to power your lecky car, that'll lower the level in the bucket. Windy, Sunny, and Nuky can't put any more in - so it's down to Gaz and Chalky to put in as many extra balls as you took out. Yes, you'll have some green, yellow, and white balls - but ALL the ADDITIONAL balls going to replace what you took out will be blue or black. There will be blue and black balls going into the bucket that would not have gone in if you hadn't taken a big handful - but not one EXTRA green, yellow, or white ball will have gone in to replace what took.
So you can try and justify it any way you like, for the foreseeable future in the UK - if you run an electric car then it is not "green". The lecky you use will be a mix, but every kWh of lecky you take will have caused an additional kWh of output from gas or coal fired plant.