When I priced out solar, it was never sold as a backup solution; it was apparently intended as a 'sell back to grid' solution. To add a battery effectively doubled the cost.

When I had solar, ~10 years ago, it was similar. We had net metering, no batteries, and zero backup if the grid went down. This was in an area where that rarely happened anyway, so I didn't really care. It'd be easy to add batteries, though.

But net metering is becoming less common, and if you can't sell to the grid at retail, then it'd make sense to store it locally. In some cases, it can also make sense to use batteries even without solar. A good sized battery can keep your refrigerator running for days, which is useful for areas prone to weather related outages. It can also easily fully power the electronics on a gas oven for a long time. And honestly, a big battery these days isn't even that expensive.

And if that isn't enough, some batteries can be topped up with the power from a large battery EV. DCFC tends to come back before a lot of residential power, so this can be really useful.

Unfortunately, that 'sell back to grid' price is often only a small fraction of the ~17 cent/kWh purchase price from the grid. The battery is less for backup but is instead to help make economic sense for your home, by storing the excess you produce when it is sunny...