Tesla Motors says its all-electric Model S sport sedan will join the Roadster in late 2011 and be sold initially with battery packs offering 165 and 230 miles of range.

But Tesla has also said that the $57,400 base-price car (which is eligible for a $7,500 federal tax credit) would have a third option a year after launch: a battery pack that will give it an impressive range of 300 miles between charges. But just how big would that pack have to be?

In an interview, J.B. Straubel, Tesla’s chief technical officer, said that the size of the pack has not been finalized. But, he said, a range of 85 to 95 kilowatt-hours is possible. “We hesitate to print a number,” Mr. Straubel said. “It would be the biggest pack on the market, and we’re designing and building it ourselves.”