Energy entrepreneur T. Boone Pickens takes part in a panel discussion titled "The End of Energy Scarcity?" at the Milken Institute Global Conference in Beverly Hills, California.
They explained why on stage during their talk at the SkyBridge Alternatives conference in Las Vegas on Wednesday.
"Tesla is this big car company," Zell said.
"You and I are subsidizing millionaires to buy these cars ... I don't understand it."
Zell was talking about the government subsidies Tesla receives in order to make its now world-famous electric cars sold by PayPal cofounder Elon Musk.
Zell and Pickens are not the first people to point out Tesla's inefficiencies.
Short-seller Jim Chanos announced a short position in the stock last week, partly due to risks the company itself has disclosed - being unable to deliver vehicles on time, for one.
"There is no substitute for cost benefit analysis," Zell continued. "Ultimately if we keep plowing money into rich people buying Teslas I don't think we're going to solve the problem."
Pickens agreed, and made an overall point about renewable energy.
"Let them get down there and compete like everybody else," he said.
The men went on to make a few backhanded compliments directed at Elon Musk. They said he might be the smartest man in the world, the best dealmaker and the best salesman.
"He's got something that's very interesting, but I'm not very interested in it," Pickens said in closing.