I do my calculations the other way round, in cost per mile.
At 60p/KWh (which is what I'm expecting my rate to be this winter), my Mini EV will cost 16p/mile in winter, 13p in summer, and actually it's much less than that in practice as there's generally excess production from our solar panels we can use. at £1.84/litre (current price of diesel round our way) the Defender costs 30p/mile.
When the 'leccy goes up to £1/KWh the Mini will still only cost 26p/mile to run in winter and we'll still have the solar panel output.
The big problem with trying to sell home produced solar energy back to the gris is that it tends to get produced at times of low demand (summer daytimes) and there's no means of transforming back up to the 40kV of the transmission network, so it's only available within your local distribution network.
Much better strategy is to increase local battery storage capability to use it oneself.
