Your price is way off - a PowerWall costs about $6,000; not $15K.
I am talking about the grid storage system in Australia - it earns over $70,000 PER DAY. They are going to build more of them. That says it all.
https://electrek.co/2018/09/24/tesla...-cost-revenue/
$66 Million / $34 Million = about 2 years ROI
And they don't need to build / operate a peaker plant - which is a huge savings.
Keep in mind, grid storage will help us NOW, with the generation mix we already have. And it will help us move to 100% renewable energy.