Electric cars are already cheaper to own and run than petrol or diesel cars in the UK, US and Japan, new research shows. The lower cost is a key factor driving the rapid rise in electric car sales now underway. At the moment the cost is partly because of government support, but electric cars are expected to become the cheapest option without subsidies in a few years. The researchers analysed the total cost of ownership of cars over four years, including the purchase price and depreciation, fuel, insurance, taxation and maintenance. Pure electric cars came out cheapest in all the markets they examined. Pure electric cars have much lower fuel costs - electricity is cheaper than petrol or diesel - and maintenance costs, as the engines are simpler. In the UK, the annual cost was about 10% lower than for petrol or diesel cars in 2015, the latest year analysed. Hybrid cars which cannot be plugged in and attract lower subsidies, were usually a little more expensive than petrol or diesel cars. Plug-in hybrids were found to be significantly more expensive. "We were surprised and encouraged because, as we scale up production, [pure] electric vehicles are going to be becoming cheaper and we expect battery costs are going to fall," said James Tate, who conducted the research. At current rates, sales of electric cars could outstrip diesel cars as early as May 2019.