SAN FRANCISCO (MarketWatch) — Tesla Motors Inc.’s pricey Model S sedan was the best-selling electric car in the U.S. in December, beating the cheaper leaf from Nissan Motor Co. for the first time, according to one estimate.

Tesla TSLA, +4.42% , which in October launched a souped-up version of the Model S, sold 3,500 cars last month, according to estimates by Inside EVs, a website that tracks electric-car sales and news.

Unlike other car makers, Tesla does not report its monthly unit sales. It only reports quarterly deliveries with its quarterly financial results. It is next expected to report on Feb. 18, according to FactSet. Tesla has not yet said when it will release fourth-quarter earnings.

That means there is plenty of room for disagreement over just how well the Model S sold last month. Analysts at Cowen, for instance, estimate Tesla sold 1,900 Model S in December and 16,500 of the luxury sedan in all of 2014. Industry tracker Motor Intelligence estimates Tesla sold 1,600 Model S in December, bringing 2014 sales to 18,750.

Editor in Chief Jay Cole said in an email to MarketWatch that InsideEVs tracked U.S. owner production and delivery dates in December and those were “much, much greater” than in November. Moreover, InsideEVs also got reports “from the ground” of hundreds of cars being delivered per day through the holidays.

“Tesla is a pretty small piece of the automotive pie, relatively speaking, so I don’t think the other industry groups follow them too closely,” Cole wrote. “For us, Tesla is a big part of a very small segment, so we do.”

Nissan 7201, +1.31% on Monday said it sold 3,102 Leafs in December. The Leaf, which has a sticker price of $30,000, was the best-selling electric car in 2014, with 30,200 units sold, Cowen said.

Teslas are far pricier. The most basic version of a Model S costs $71,000. The Model S P85D, with dual motor, all-wheel drive and other perks, starts at $104,500, according to Tesla’s web site.

One electric-car model that saw a drop in December sales was General Motors Co.’s GM, -1.31% Chevrolet Volt. Some 1,490 units were sold, 38% fewer than a year earlier, as customers appeared to be waiting for the 2016 model, expected to debut next week in Detroit and hit dealerships in the second half of the year. The Volt sells for $35,000.

Some 118,259 electric cars were sold in the U.S. last year, 18% more than in 2013, the Cowen analysts said.

Electric-car sales, however, are still a sliver of the overall U.S. car market, which also boomed in 2014. U.S. car sales rose 6% on year in 2014, with 16.5 million vehicles sold, the highest annual unit sales since 2006, according to analysts at Stifel. They estimate sales will climb to 17 million vehicles this year, in part because of lower gas prices and low interest rates.

For its part, Tesla is hoping to start producing a mass-market electric car in the next two to three years, and the debut of a SUV, the Model X, is expected later this year.

Tesla shares have lost 5.2% in the past three sessions, but are up 41% in the past 12 months.