I'm pretty sure the base load (which what you're drawing from off of peak hours) is largely produced by fossil fuel generation because those sources don't have the variability of renewables (I don't think a lot of solar is being produced at night...).
Solar is a terrible reference for renewable as the US has 4x as much wind and hydro power as solar, which makes up barely 1% of generation. Nuclear makes up 9.6%.
These 3 are clear base load because they don't turn off.
Yeah it seems like, in Boston at least, grid carbon intensity peaks between 12 and 8pm and is at its lowest around 4am. It's probably on a case by case basis though -- if you live in an area that still burns a lot of coal, your base load is likely to be pretty dirty (though of course in that case it probably doesn't matter when you charge your car, unless there happens to be a lot of solar as well).
