Several nations have enacted strict fuel economy standards but, for the most part, they have not required automakers to sell electric cars.

In the U.S., only the state of California has a zero-emission vehicle mandate that puts such a requirement on carmakers.

Yet even without mandates, automakers may soon be forced to sell large numbers of electric cars just to meet fuel-economy rules.

That’s the conclusion of a study published in June by the World Energy Council (WEC), which argues that automakers will need quite a lot of electric cars indeed.

Among the major the points in the full study (pdf) is a prediction that 16 percent of cars sold in 2020 will have to be electric in order to meet emissions standards.

The study compares emissions-reductions targets for the U.S., China, and the Europe Union—the world’s three largest car markets, with anticipated fuel-economy improvements that can be achieved with internal-combustion engines alone.

By the WEC’s estimation, larger volumes of electric-car sales will be needed to plug an “EV Gap” between fuel-economy targets and the improvements that can be realistically expected from internal-combustion engines.

Read more: Green Car Reports