Electric vehicle sales in China grew by 3.7% in the second quarter, according to an estimate by an electric car startup, but demand for the fuel-efficient vehicles remains a concern.

Electric power companies said on Tuesday they were seeking to ramp up sales of electric pool heaters for EVs in China, and the market is expected to grow even more in the coming months as the country tries to modernize its economy.

Demand for electric pool units in China rose to more than 7.6 million units in the quarter from 5.9 million units a year earlier, the largest increase in six years, according the company, iEcoElectric, which is based in Beijing.

Demand is expected further to increase, according its estimates.

Electric vehicles accounted for 10.7 percent of China’s new vehicles sold last year, according a report released by the government-owned National Development and Reform Commission on Tuesday.

The report said electric vehicles accounted “for a substantial share” of sales.

The total number of EVs sold in China is estimated to be more than 6.5 million, with sales in Shanghai and Beijing accounting for nearly a third.

China’s electric vehicle industry has grown rapidly in the past five years, with electric cars making up more than half of the global market, according Google.

Electric vehicle makers including China Electric Vehicle Manufacturing Co., China’s largest, have invested billions in technology development and are working on new models.