Tesla is making money off cars its competitors aren't selling

• Some of Tesla's $25.5 billion in second-quarter revenue came from the cars competitors didn't sell.
• Tesla, which sells only electric vehicles, always has an excess of credits.

Tesla's record second-quarter revenue was partly driven by something unusual: all the electric vehicles its competitors didn't sell.

Elon Musk's electric car company has always been able to rely on under-the-radar revenue generation from selling its extra regulatory credits to car companies whose fleets don't meet emissions requirements.

Because Tesla's vehicle sales are 100% electric, it always has an excess of credits — awarded to companies that surpass their regulatory requirements — and can sell them for a nice profit.

That's a helpful leg for Tesla even as profits fall.

As the EV market gets tougher for legacy car companies, it appears that these competitors are relying more heavily on Tesla's credits to help them avoid hefty penalties for not meeting requirements.

Tesla just about doubled its regulatory credit revenues from the first quarter of this year, recording $890 million in credit sales in the second quarter

This bump helped Tesla deliver a record $25.5 billion in revenue in the April to June period, beating analyst expectations by about $1 billion.

It's the bright spot Tesla needs after a slow start to the year. While the company outperformed on revenue generation in the second quarter, its $0.52 earnings-per-share fell short of analyst estimates of around $0.60 per share.

Tesla's stock fell as much a 7% in after-hours trading Tuesday following the report.