Just about a week after the company's press conference announcing a software update for its Model S fleet, Tesla Motors (TSLA -3.55%) CEO Elon Musk is announcing another event. On April 30, at 8 p.m. PST, Tesla will introduce a "major new Tesla product line" that is "not a car," Musk said on Twitter Monday morning.

While we can't be certain, the new product line Musk is referring to is probably the company's "home battery" the CEO teased during its fourth-quarter earnings call in February.

"We're going to unveil sort of the Tesla home battery -- consumer battery -- that will be for use in people's houses or businesses, fairly soon," Musk explained. "We have the design done. And it should start going into production probably in about six months or so." 

Musk went on to say that Tesla was trying to figure out a date to have the product unveiling and noted that it will probably be "in the next month or two." The timing of an April 30 announcement coincides with Musk's timeline for a battery unveil.

What would a home battery be for? To get a better understanding of the product's purpose, look to solar company SolarCity (SCTY.DL), where Musk is Chairman. SolarCity CEO Lyndon Rive, Musk's cousin, said last year that the company would eventually sell every home solar system with a battery-storage system. Musk has said that Tesla will supply at least some of the batteries SolarCity will need to offer this service.

SolarCity's DemandLogic energy system pairs solar panels with Tesla batteries. Image source: SolarCity.

Tesla's next frontier
Tesla is taking its foray into energy storage seriously. Musk's right-hand man, Tesla Chief Technology Officer JB Straubel, who leads Tesla's energy storage and battery efforts, has said that Tesla is as much "an energy innovation company" as it is a car company.

SolarCity solar panels. Image source: SolarCity.

Just how serious of a business does Tesla imagine energy storage to be for the company? The best measurement we have of Tesla's expectations is a comment from Musk that as much as one-third of the Gigafactory's output will be intended for stationary storage. With the company planning to produce 50 gigawatt hours of pack output per year by 2020, therefore, Tesla may be aiming for 16.5 gigawatt hours of this pack output to be used for energy storage -- or four times the watt hour storage used to support Tesla's 2014 vehicle production.

For Tesla's energy storage business to take off, Tesla is betting costs for solar panels and lithium-ion batteries will continue to fall. The lower the costs for solar panels and batteries, the larger Tesla's addressable market for a home battery system.

Of course, it's still not clear that a home battery will actually be the major new Tesla product line unveiled on April 30. But we do know a home battery is coming.