For years there has been a big split between investors who expect Tesla (NASDAQ: TSLA) stock to go to the sky – and those who think it is due to crash down to earth at any moment.

In many ways the past year has been challenging for the company. Despite that, Tesla stock – already pricey a year ago – has risen another 50% over the past 12 months.

Could there be more to come – and ought I to buy some for my portfolio?

Tesla: more than cars?

The crux of the debate boils down to one question: what is Tesla, besides a carmaker?

That is not to understate the importance of the car business currently. Tesla shifted over 358,000 vehicles in the first quarter of this year. In the quarter, 73% of its revenues came from its automotive division, and many additional services revenues flow from it.

But while the automotive division is big business — $16bn of revenues in a single quarter – that hardly justifies the company’s $1.2trn market capitalisation in my view.

General Motors has a massive car business that sold more cars than Tesla in the quarter, but its market cap is $71bn. Ford, with its $51bn market cap, also outsold Tesla.

Car sales may grow over time, although the past year has seen mixed signals on that front in an increasingly competitive electric vehicle market. Car sales volumes in the first quarter did grow 6% year on year, but the overall sales trend for Tesla over the past year has been negative. Last year, volumes fell 9%.

The only way to justify that valuation, then, let alone a higher one, is the prospect of the business growing in other ways.

New possible avenues for growth

There is another big business Tesla already operates in: power generation and storage.

That generated $2.4bn of revenue in the first quarter. Alarmingly for a business seen as having strong growth prospects, that actually represented a 12% fall compared to the same quarter last year.

Beyond that, the company has a lot of other business ideas that could potentially grow big in future. They include self-driving vehicles, humanoid robotics, and AI.

Tesla has a credible claim to do well in each of these areas.

It has already gained relevant experience and skills for each through its car business. It has also demonstrated an impressive capacity to innovate, disrupt an existing sector, and ramp up sales massively in a short number of years.

Lots of ideas, but lots still to prove

However, just because it has done that before in the automotive space is no guarantee it will succeed in the new business areas.

While it has relevant skills, so do a lot of other companies. Some are already far ahead of Tesla in rolling out their businesses, notably in AI and self-driving taxis.

Tesla does not yet have a commercial business in these areas, let alone one that has proven profit potential.

If it gets there at scale, Tesla stock could yet soar. But with high costs involved and arguably no strong competitive advantage compared to some rivals, it might never get there.

So, for now, I see the stock as badly overpriced – and have no plans to invest.