How close plug-in electrified vehicles (PEVs) get to true zero emissions depends on electricity power sources in that country, according to data compiled by Bloomberg New Energy Finance.

PEVs are producing, on average, about half the amount of pollution as a gasoline or diesel car, but there is great variation in that range. If much of the energy is powered by solar, wind, or nuclear, it’s much cleaner than if the electricity is being generated by coal-powered plants.

China, while being the fastest growing PEV sales market, will need to source more of its power from clean energy to see a difference. Its PEVs were only 15 percent cleaner than petroleum-powered cars last year, according to the data. Most of its electricity is coming from coal-powered plants.

“In countries with large amounts of coal in the generation mix, EVs still look better, but the benefit is smaller and depends on when and where charging takes place,” said Colin McKerracher, head of advanced transport analysis at Bloomberg New Energy Finance.

The Bloomberg data coincides with a Union of Concerned Scientists study. Both analysts say that PEVs are 40 to 50 percent cleaner than vehicles fueled by gasoline or diesel. Along with sources of power, each country’s PEV-to-fossil-fuel ratio will also be affected by a sales increase of fuel efficient gas engine vehicles as national mandates increase in the coming years.

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PEV sales will make more of a difference in each country’s efforts to reduce carbon emissions. Sales were up 44 percent in the first half of this year, and may end the year at a record 647,000 worldwide, according to BNEF’s forecasts. Automakers will be rolling out several new models in the coming model years, and production capacity for Volkswagen and other companies is slated to increase significantly within the next decade.

Energy sources vary by country. Norway generates almost all of its electricity from hydroelectric dams coming from mountain rivers. In France, more than 90 percent of power comes from nuclear and renewables.

China, India, Germany, and Spain, are seeing solar and wind plants produce more and more electricity. Japan has been going in the opposite direction since its nuclear power plants were taken out after the devastating earthquake in 2011. Its emissions are increasing as more plants are being built that are powered by coal and natural gas.

No matter how the power is being produced, selling more PEVs is helping reduce emissions in that country. Overall, energy is getting cleaner and PEVs make more of a contribution than fuel efficient gas engine cars.

“Essentially, the generation mix gets cleaner faster than internal combustion engines can improve over the next 25 years. The future is electric,” McKerracher said.