It’s about 7k more to add 4.8kwh battery now and no maintenance or perpetual cost to keep it running.
Also no noise.
And it’s not about one sunless day. It’s about every day.
I only need one more battery to make it through a very bad day, and then I’m sorted for the next ten years.
What about multiple days of bad weather and the grid is down (stolen cables, substation blew up, etc)? That extra battery is only going to buy you one more day and buying a week's worth of batteries doesn't make financial sense.
When I put solar in I'll still keep the generator for backup.