Last year Tesla took in $413.3 million in revenue — but a big chunk of that wasn't from selling cars. The electric vehicle manufacturer made about 10 percent of its income last year — $40.5 million — from selling emissions credits to other automakers, reports The Wall Street Journal. Twelve states have regulations requiring vehicle manufacturers to sell a certain amount of zero-emissions vehicles each year. Companies that exceed these quotas can stockpile or sell the extra 'credit' that the company earned that year. Companies that don't reach the quota face fines or sales limits — unless they purchase excess credits from a competitor.

Bloomberg reported on the emerging zero-emission credit marketplace after California's stricter regulations went into effect last year; the other 11 states follow the regulations set by California as well. For companies such as Tesla and Nissan that already have zero-emissions vehicles in their fleet, selling credits is a burgeoning opportunity. Credits earned between 2009 and 2011 will eventually reduce in value, but beginning last year credits no longer have an expiration — which means that companies can begin to stock up.

The credits system is variable: vehicles with a greater range and vehicles that can recharge quicker are more valuable. With a 100-mile range, Nissan's Leaf receives three credits, while the long-range and quick-charging Tesla Model S is worth seven. Tesla isn't yet required to meet the emissions standards — it only sold 2,560 vehicles last year, enough to fall outside of the regulation's scope — but each sale still accrue credits for the company.

By 2025, manufacturers that sell 20,000 or more vehicles each year will be required to have 15 percent of their fleets be emissions free. Even if manufacturers currently meet that standard, they may fail to earn enough credits if their zero-emissions vehicles don't sell. Honda purchased credits from Tesla in 2008 and 2010 after its FCX Clarity sedans didn't sell in as large quantities as it had expected.

In spite of revenues in excess of $400 million, Tesla posted a loss of $396.2 million for 2012 — and the credit market may not stay around for long. As technology improves for electric vehicles and increasingly stringent laws promote adoption by automakers, credits will eventually flood the market and devalue. Until then, California's regulations are incentivizing a cleaner future, and Tesla's vehicles are in many ways setting the bar.