I actually talked about this in a previous comment:
What about on-site battery?
In any case, $15k is still a bit too steep for me, not because I can't afford it but because I want something that'll pay for itself in less than 5 years. My electricity bill is about $150/mo. so a solar roof doesn't make sense financially, even if I'm able to sell the excess to the grid.
Typical systems don’t have a battery, nor do they need one.
The grid is used as the battery.
An off-grid solar installation is significantly more expensive.