Low oil prices and cheap gas are cutting into the demand for electric cars and solar power, if analysts are to be believed. That's no good if you're the largest shareholder of Tesla and SolarCity, and some back of the napkin math says Musk's pocket is about $1 billion lighter.

Musk owns over 28 million shares in Tesla, which peaked at around $284 in September and is now floating around $200 today. He also owns almost 21 million shares of Solar City, which topped out in February at $86 and is trading today at just over $50.

According to Marketwatch:

On Nov. 26, the day before a decision by the Organization of the Petroleum Exporting Countries to stand pat on production suggested the group preferred to defend its market share than try to support prices by cutting output, Musk's combined Tesla and SolarCity holdings were valued at around $8.2 billion. They are now more like $7 billion.

Since that OPEC meeting, Tesla shares declined 16 percent and Solar City dropped by 11 percent, and it's even lower now. But really, what's a few hundred million between billionaires?