> you could build enough solar panels that solar output at 18:00 matches power demand

No you could not. For half the year the sun has set by 18:00.

I mean in the dead of winter, yes. For six months of the year? Definitely not.

	▲	perilunar 24 minutes ago \| parent [-]
		Definitely so. Unless you are on the equator, the sun is up for less than 12 hours a day from the autumnal equinox to the spring equinox. The sun will set before 18:00 local solar time. So apart from funkiness with time zones and summer time (which extends a couple of weeks past the autumnal equinox in Aus), yes, roughly half the year.