The existing power grid basically with maybe some local improvements. An ev the charges while parked averages to less power than an electric kettle.
If you want a harder problem, it’s electric building heat in cold climates.
That said my basic calculation for New Zealand is additional 50% is needed. Probably easy peasy if we get over lobby given how much existing hydro we have and capacity for sun and wind.
Using Chinese offshore wind turbine costs my estimate was 1/3rd of some major railway project cost we are considering…