No matter how you look at it, electric vehicles aren’t going to replace the world’s internal combustion engine-driven cars and light vehicles overnight. But what about 20 years from now?

According to a Monday report from the Institute for Energy Efficiency (IEE), even by 2035, electric vehicles will make up no more than 30 million of the 261 million light-duty cars, trucks and vans on the road, or about 12 percent of the world’s fleet -- and that’s a best-case scenario.

IEE puts its mid-range scenario at 25 million EVs by 2035, adding up to one in ten vehicles on the road. But its most conservative (that is, pessimistic) scenario puts EV penetration at only 5 million vehicles, or about 2 percent of the world’s fleet by then.

Battery technology advances are the key variable separating IEE’s low-adoption and medium-adoption scenarios. In fact, IEE names its mid-range, 25 million EV scenario the “Advanced Battery scenario” to distinguish it from its low-range case, which it derives from the U.S. Energy Information Administration (EIA)'s Annual Energy Outlook (AEO) 2012.