Bitcoin traded above $39,000 for a second day on Monday after Elon Musk tweeted Tesla would accept payment in cryptocurrency again once mining can be done using cleaner energy.

"When there's confirmation of reasonable (~50%) clean energy usage by miners with positive future trend, Tesla will resume allowing Bitcoin transactions," the billionaire said in a tweet.

Barely three months later, Tesla made a U-turn and stopped payment in bitcoin because of how energy-intensive the mining process is

"We are concerned about the rapidly increasing use of fossil fuels for bitcoin mining and transactions, especially coal, which has the worst emissions of any fuel," Musk said in a tweet at the time

It isn't clear yet how Musk will gather data to assess bitcoin's clean energy use