People look at a Tesla Motors vehicle on the showroom floor at the Dadeland Mall in Miami on Feb. 19, 2014. Joe Raedle/Getty Images

Tesla Motors Inc. sold 1,652 Model S luxury electric cars in the U.S. last month, 50 percent more than in May 2014, according to an estimate from WardsAuto. But analysts point out that the number is a ballpark figure based on state vehicle registrations that shouldn’t be used to measure how well the company is achieving its goal of selling 55,000 cars worldwide this year.

Meeting sales targets is vital to the company’s goal of delivering 500,000 vehicles per year by 2020, up from 31,655 last year. If Tesla succeeds, it would be the fastest ramp-up in automotive history, and both boosters and naysayers are closely tracking sales numbers to measure this progress.

“It’s good to see growth in the U.S. on a year-over-year and quarter-over-quarter basis, but I would caution against using estimates like this,” said Benjamin Kallo, an analyst for clean technology at Robert W. Baird & Co. “Accuracy is not 100 percent, and we’ve seen third party services get way off on some of their estimates. You have to take all of this with a grain of salt.”

The WardsAuto estimate released Tuesday says Tesla sold 1,652 Model S cars in May, up from 1,100 in the same month last year and 1,377 in April. That puts the number of U.S. Model S sales in the first two months of the current April-June quarter at 3,029. Unlike other automakers, Tesla doesn't report monthly sales, so all monthly estimates are unconfirmed by the company.

Earlier this year, the automaker began reporting quarterly sales at the end of each quarter in order to avoid the speculation that often took place between the end of the quarter and the release of the number in the earnings statement weeks later. The next official Tesla sales figure will be out at the end of June.

Tesla sold 10,030 Model S cars globally in its first quarter, including 1,400 units whose deliveries were delayed due to the weather and because customers were on vacation, according to the company. This means Tesla needs to average 14,990 global unit sales per quarter for the rest of the year to meet its annual target. If the WardsAuto figures for U.S. sales are accurate, Tesla will unlikely hit 14,990 unit sales in the current quarter. The U.S. is Tesla’s largest market and makes up the biggest portion Model S sales.

Tesla has said it would sell more cars toward the end of the year than at the beginning, especially after the planned rollout of the highly anticipated Model X crossover SUV, so it doesn’t need to hit the 14,990 number in the second quarter -- it can make up for that in the second half of the year. But Tesla will need to sell more cars in the April-June quarter than it did in the first three months of the year (10,030 units) to avoid any speculation of falling demand.

“From what we can tell demand is very solid,” Kallo said.

Tesla shares were trading up a slight 0.29 percent to $249.07 on Wednesday. Tesla Motors is holding its annual stockholder meeting on Tuesday in Mountain View, California. The event starts at 2 p.m. PDT. Listen to the live webcast here when the event begins.