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Tesla reveals its mystery 'Missing Piece' Thursday night

An invitation for an event this week touts a new product line from Tesla Motors -- widely believed to be a home battery backup system.

Tesla Event invite
Tesla's event invite notes the 'Missing Piece' and shows a vague image that could be a battery casing. Tesla


Tesla Motors sent invitations out for a Thursday night event at its Southern California design center referring to 'The Missing Piece,' following up on an earlier tweet from the company's CEO, Elon Musk. Although Tesla preserves its usual air of mystery about the event, Musk noted that it would not be a car.

Musk's tweet described a "major new Tesla product line." Based on that tweet, CNET reporter Nick Statt noted other evidence that Tesla may be announcing a backup battery pack for homes.

The idea of a battery pack for a home is not too far-fetched. Nissan is currently developing a system where its Leaf electric vehicle could serve as an emergency power supply for homes affected by a blackout.

According to the US Energy Administration, the average US home uses 30.3 kilowatt-hours of electricity per day. The Tesla Model S P85D comes with an 85 kilowatt-hour battery pack, enough electricity to keep that typical home fully powered for almost three days. If Tesla were to package the Model S' battery as a separate unit, with conversion hardware to connect to a home, the unit could keep appliances and lights running when your supply fails.

Given that a home battery pack would not have to be restricted in weight or size, Tesla could easily increase the capacity.

Beyond emergency backup power, however, home batteries of this capacity could serve to balance out the grid's electrical load. Power companies run enough generating capacity to supply peak load power, which typically occurs in the afternoon or early evening, supplying electricity for air conditioners, lights and appliances. Minimal loads occur during late night and early morning hours when most people are asleep. Because power plants can't easily change output, utilities offer off-peak electricity at much lower rates due to the extra supply.

A Tesla home battery could charge up during off-peak hours, then supply its electricity to homes during peak hours. Home owners would see savings in their electricity bills, and power companies would see less peak demand, balancing out electricity usage and putting less stress on power plants.

We won't know the whole story until Musk takes the stage on Thursday night, at an event scheduled to start at 8 p.m. Pacific Time. CNET will be on hand to offer full coverage.