The waiting list is piling up for this hot piece of technology from Tesla Inc. (TSLA) .

No, it's not the Model 3 - or any other car model, for that matter.

I'm talking about Tesla's Powerwall.

The Powerwall is a household battery that's designed to provide backup power or off-grid energy for homes with solar. The idea is straightforward - solar panels (or Tesla's new Solar Roof) produce energy during daylight hours, with excess wattage going to the home's Powerwalls.

Then, at night, when your solar array goes dark, the battery powers the house.

Theoretically, having a Powerwall means that residential customers can separate from the grid, or enjoy continuous power during outages.

(And in states where utilities don't offer net metering for solar customers, it can dramatically reduce power costs.)

Put simply, it's an attractive offering for anyone drawn to solar.

And Tesla can't keep up with demand right now.

In their full-year 2017 update, Tesla reported that the majority of Solar Roof customers are opting for at least one Powerwall. They also reported that "solar deployments were affected by the short supply of Powerwalls for customers who wanted solar plus Powerwall in their house."

Keeping up with demand for the Powerwall is surely complicated by the fact that Tesla is focused on ramping up its Model 3 production, but the fact that Tesla's energy arm has a handful of supply-constrained products is frankly a good problem to have.

Tesla's name change last year from Tesla Motors to Tesla Inc. was a signal that the firm is an energy company, not just a car company. And the line of residential, commercial and utility customers waiting to have solar arrays, Powerwalls and commercial energy storage installed is a clear indication that Tesla's energy business has the potential to move the needle meaningfully in the near term, despite being a comparatively small contributor to revenues today.

None of that changes the fact that Tesla's price action remains in a shallow downtrend right now, but it's another well-defined catalyst that could help buyers retake control of shares in 2018.

For now, the key price level to watch in Tesla is right around $350. If shares can muster the strength to push back through that price level, which defined the top of Tesla's price channel, then we've got a signal that the bulls are back in charge and that it's time to join them.

Until then, patience is a virtue, both for Tesla bulls, and consumers waiting for Powerwall deliveries.

This article is commentary by an independent contributor. At the time of publication, the author held no positions in the stocks mentioned.