Battery electric vehicles have the lowest cost of ownership when compared to other low-emission vehicles as well as conventional vehicles, according to researchers at the University of Leeds in the United Kingdom.

The researchers said previous studies have not compared the total cost of ownership across more than two vehicle markets. They said there has also been a lack of research into historic costs.

The study compared the ownership costs of hybrid electric, battery electric, plug-in hybrid, and traditional gasoline-powered vehicles. It looked at the time period between 1997 and 2015 and the markets in Japan, the United Kingdom, and California and Texas in the United States.

Researchers compared the ownership costs of the Toyota Prius hybrid and plug-in hybrid models as well as the Nissan Leaf battery electric vehicles. The conventional vehicles used for comparison were the Toyota Corolla in Japan and the Ford Focus in the U.K., since both models were considered to be comparable to the Prius.

To compare costs, the researchers developed a formula which took into consideration the higher initial ownership costs of low-emission vehicles. It also included factors such as the depreciation rate, annual fuel prices, fuel efficiency, maintenance costs, insurance, taxes, and any financial incentives for owning a low-emission vehicle.

Researchers concluded that the ownership costs of hybrid and electric vehicles are all lower in 2015 than they were in 1997. In each market, electric vehicles were found to have the lowest annual ownership cost among all vehicle types.

"We were surprised and encouraged because, as we scale up production, [pure] electric vehicles are going to become cheaper and we expect battery costs are going to fall," said James Tate, one of the researchers in the study.

Plug-in hybrid vehicles had the highest ownership cost in each market except Japan, where this type of vehicle had the second lowest ownership cost. Hybrid vehicles had the highest ownership cost in Japan and the second highest cost in the other markets.

The larger battery and other vehicle features make low-emission vehicles more expensive to purchase at the outset, although this price premium has been reduced as powertrain technology improves. Low-emission vehicles have also proved cheaper to run and maintain.

Electric vehicle sales have increased in recent years, but remain a small share of overall sales. According to the site InsideEVs.com, 173,941 electric vehicles have been sold in the U.S. in the first 11 months of the year – already exceeding the 2016 total of 158,614. It calculates worldwide sales to be 893,370, up from 777,497 in 2016 as a whole.

The site determines that the top five bestselling models—the Tesla Model S, Chevrolet Bolt EV, Toyota Prius Prime, Chevrolet Volt, and Tesla Model X—have accounted for more than half of all electric vehicle sales in the U.S. this year. Volvo, which announced in July that all its vehicles will be hybrids or electric vehicles starting in 2019, have only made up about 1.3 percent of the year's EV sales.

A recent study by the University of Michigan Transportation Research Institute to compare emissions from gas-powered vehicles and electric vehicles found that the generation of greenhouse gases varies widely depending on available fuel sources. Researchers took into account the emissions produced in the extraction, refining, transportation, and combustion of oil and gasoline as well as the extraction and delivery of fuel to run electric power plants.

UMTRI determined that gas-powered vehicles in the U.S. would need to get 55.4 miles per gallon to produce lower emissions levels than electric vehicles. The study also found that in countries with cleaner power sources such as Albania, which derives most of its electricity from hydropower, gas-powered vehicles would need to get more than 5,000 miles to the gallon to produce lower emissions levels than electric vehicles.

The University of Leeds study concludes that government support for low-emission vehicles needs to address the financial barriers created by the higher initial upfront costs. It also advocates for the phasing out of incentives for low-emission vehicles once cost parity has been reached with gas-powered vehicles.