I live in central Massachusetts and have a roughly analogous rooftop solar system as Okiedo (formerly SolarCity, now Tesla, installed by previous owner in 2014). Massachusetts is roughly at the same latitude as Portugal, to compare with Europe, whereas Germany, which has a lot of rooftop solar generation, occupies latitudes between Calgary and Edmonton. So it's possible in the north, just not necessarily all year long. Here, our panels generate plenty to run the house and charge a couple of EVs from March to November. [We don't own EVs since my wife and I both needed to buy new cars in 2015 while we were still living in an apartment in Boston without the ability to charge, but we will soon.]

As for winter, well last night 15 inches of snow fell on the panels so they're not producing anything today. So the grid supplies it. ISO New England is ~50% natural gas, and ~50% nuclear/hydro/solar/wind. There'll be more off-shore wind in 2023, and more Canadian hydro soon too. In the winter, natural gas / nuclear is currently the primary source since the sun is low and rivers are frozen. I believe, though, that with more off-shore wind coming online in the next years, the % of power from natural gas won't need to rise substantially as the demand for EVs increases. But that's just a guess. 

As for rooftop solar ROI, that all depends on what you add (i.e. estimated cost increases of grid power) and subtract (i.e. state subsidies and credits) from the estimated savings. SunRun says it'd take us under 6 years to pay back a system in MA, and I don't think there is any state anywhere anymore where it takes 20 years. 

Finally, I think 50% EVs by some random date (depending on the country or state) is largely aspirational. And I believe it's usually 50% of the fleet rather than even 50% sold or 50% rolling, no? Converting the rolling stock will take some time, time enough to solve some of the problems we might encounter with generation. But the trend is definitely set, and similar things are possible, and the demand is coming, all around the country.

Sources:

https://www.iso-ne.com/about/key-stats/resource-mix/

https://www.sunrun.com/solar-lease/cost-of-solar/solar-energy-economics