> Factor in lower generation in December, you'd need 10 times that amount of solar, about 70km by 70km, 5000 square km.

Or, convert the solar energy to heat, then back to electricity at 40% round trip efficiency. Needs only 2.5x the solar (or less, because some solar goes directly to the users without long term storage). The heat is very storable at scale, even for seasonal storage.