Elon Musk's personal wealth increased by more than $36 billion Monday when Hertz Global Holdings ordered 100,000 Tesla electric cars, with the latest filings also showing Musk had a fresh tranche of options in his massive pay package.

Tesla shares increased 13% on news of the Hertz order, bringing the company's market capitalization above $1 trillion.

This is the largest single-day increase in the Bloomberg Billionaires Index's history, surpassing Chinese tycoon Zhong Shanshan's $32 billion gain last year when his bottled-water company, Nongfu Spring Co., went public. 

Musk's net worth, which has surpassed Exxon Mobil Corp. and Nike Inc., is now almost $289 billion.

Roughly two-thirds of Musk's net worth is directly tied to shares and options in Tesla, the electric-car firm in which he is a co-founder and CEO.

Musk is fast eclipsing his fellow mega-billionaires in terms of wealth. Jeff Bezos of Amazon.com Inc. is in second place with $193 billion, according to the Bloomberg index.

Musk's meteoric rise in fortune comes as Democratic senators, with United States President Joe Biden's assistance, work on a plan to tax unrealized gains of the very wealthy Americans. 

Senator Ron Wyden, a Democrat from Oregon, is outlining the so-called billionaires' income tax, which would apply to anyone with $1 billion in assets or three consecutive years of income of $100 million or more.

Musk's fortune is stock-based, which enables him to accumulate billions of dollars in paper wealth and become the world's richest person despite having little marketable assets.

According to corporate documents, he receives no compensation from Tesla and has pledged a portion of his ownership as collateral for personal loans. 

He testified before a federal jury in 2019 that despite his multibillionaire status, he was cash-strapped. He declared last year on Twitter that he intended to sell his properties and "almost all" of his physical properties.

Tesla stock have risen in recent weeks even without the Hertz deal. The stock price of the automaker is up 45% this year, more than double the rise of the S&P 500 Index, as investors continue to reward green technologies.