The European Union’s promise to buy $750 billion of American energy imports over three years was pivotal to securing a trade deal with President Donald Trump, but it’s a pledge it will struggle to keep.

The energy deal is a centerpiece of the trade agreement reached between Washington and Brussels on Sunday, which puts baseline U.S. tariffs at 15% for most European goods. The EU has agreed to buy $250 billion worth of U.S. oil, gas and nuclear fuels per year for three years, European Commission President Ursula von der Leyen said.

The EU has pledged to buy $750 billion of energy from the U.S. in exchange for a lower rate tariff rate in its trade deal, but analysts say this is unrealistic.