Image source: Tesla Motors.

Tesla's (TSLA -1.38%) Powerwall 2 was unveiled in an event on Friday, and it brings energy storage a step closer to reality for the company's customers. When combined with SolarCity (SCTY.DL) solar panels, the energy storage system could allow a customer to generate 100% of their electricity on some days, which is the ultimate goal for Elon Musk's energy vision. 

But before we get too excited about a wall-mounted battery pack, we need to unpack what's improved and what hasn't about the Powerwall 2. 

A small step forward

When the original Powerwall was announced last year, it garnered a lot of attention in the media and from customers. 38,000 Powerwalls were pre-ordered shortly after the product was announced, but only a small fraction of those orders turned into sales. Tesla doesn't break out Powerwall unit sales, but given the Tesla throws the product in with the $150 million of quarterly sales in "service and other" -- which includes Powerwall, Powerpack, used car sales, and vehicle services -- we can deduce that there's not a significant number of sales for Powerwall 1. 

The second-generation Powerwall makes some slight improvements, increasing size and lowering cost. But the cost of Powerwall 2 is only 13% less than the previous generation on a per-kWh basis. 

Data source: Tesla Motors. Table by the author. 

What this doesn't account for is the integrated inverter. Powerwall 1 needed the battery and an inverter to turn DC power to AC and that component is now included. But we know almost nothing about it. 

Will this be Powerwall's killer feature?

Elon Musk has been arguing for months that Tesla and SolarCity need to combine to create a more cohesive energy offering. And Powerwall 2 was a chance to show that vision, but there was no integration to speak of in the announcement. 

The fact that Powerwall 2 includes an inverter was only discovered after the event when press asked Musk the question directly. But we don't have information about the system's efficiency, how it could streamline installation, or how customers will generate revenue or cost savings from the product. 

It's easy to speculate that this could be a step forward in the solar+storage future for homeowners, but we don't know enough about the product or how it works to make that determination yet. 

Why the Powerwall won't change energy...yet

Eventually, energy storage is going to be a big business around the world. But it's not yet clear that energy storage in the home will make financial sense for SolarCity or Tesla customers. Powerwall 2 didn't do anything to accelerate the justification for energy storage yet. 

For now, Tesla is more likely to build a large energy storage business serving utilities with the Powerpack product. Powerpack is more cost effective and can be used to reduce costs in other parts of the grid, like substation or transmission upgrades, that residential energy storage can't. If investors are looking for where Tesla can make an impact on energy, I would look at the Powerpack, because Powerwall 2 won't be a big seller for many years to come.