They can be depending on your needs. Lithium iron phosphate batteries are pretty cheap for their capacity. If you build your own power station with them you'd be surprised how far your money goes.
In Australia, if you have the space for rooftop solar, it's far cheaper in the long run to buy solar+battery. We did the math for our household and even if grid prices are stable (which they aren't, they're fast increasing) we're still going to make money back on the investment in less than 4 years.
Granted this includes a government rebate for the battery, but overall the prices have plummeted. Any government that isn't pushing for renewables and energy storage at this point is actively working against it's citizens.
No, and once the Solar Sharer scheme kicks in it'll be very helpful in avoiding leaning too hard on the grid in the evening after rainy or overcast days.
It's a fantastic way to solve oversupply; give it to everyone, including those who have batteries in areas where the weather restricts solar output.
