The cheapest electricity source is the one you already have: fossil.
Also, what good is the added capacity if it's being used up by bitcoin? Everything you pour into btc isn't going into an EV.
But so you think using more energy will solve our energy problems. Sorry, but that's stupid. It's like thinking using more money will solve your money issues. Or using more water will solve you water issues.
It just doesn't work like that.
You may think "but solar panels create electricity from something that comes from outside the planet, therefore it is unlimited."
And yeah, there's some truth to it.
On the other hand, if we capture that energy and turn it into heat, which is essentially what bitcoin does, we're adding heat to the planet that, as you would have pointed out, wouldn't stay here in the first place.
So yeah, unlimited energy from the sun, limited capacity of the planet to soak it up before we all die.
Seriously, proof of work must die. And proof of stake... well... as far as I understand, it's no better than boring old fiat currency.
PS: oh, and if you think that merely writing on a technology forum grants you "the ability to see the future better than the average person", boy you're mistaken. Or at least I foresee a different future than you do.