This is a roughly 12 hour reservoir:

https://www.google.com/maps/@43.9207263,-86.3978435,18314m/d...

It's a big power station, 1.8 GW, but is only providing service to portion of consumers in Michigan.

Making it 10 times bigger would probably be doable but it wouldn't be trivial.

It's also one of the better places on the planet to build a single reservoir system (otherwise you need 2 enormous reservoirs).

I wonder if the answer to seasonal variation will be to make fuel in the summer.

A larger connected world grid could solve some of the problems 10 years or so ago I read some articles of producing solar power in Africa and then using it in Europe.