Hyundai Motor and Kia Motor are posting a rapid growth in sales of hybrid sedans at home, following outstanding performances in the U.S. market.

In July, about 15 percent of Hyundai-Kia’s customers who purchased the automakers’ Sonata and K5 picked the hybrid version of the sedans, Hyundai Motor Group said Tuesday.

Their total sales of the Sonata and K5 came to 7,772 and 7,051 units last month. Among them, hybrid models accounted for 19.3 percent (1,500 units) and 10.3 percent (729 units).

Hyundai-Kia officials say more and more consumers are expressing their interest in the higher fuel efficiency and engine power of the hybrid models.

“We will be able to attain our initial goal of selling 11,000 Sonata hybrids and 6,000 K5 hybrids this year,” a company spokesman said.

Though the average price tag of the hybrids is higher by about 3 million won ($2,850) than that of ordinary gasoline models, consumers benefit from fuel efficiency of 21 kilometers per liter.

“This could make up for the higher purchasing cost of 3 million won if customers drive about 20,000 kilometers for the first five years,” he said.

According to Hyundai Motor Group, the lithium ion polymer battery pack used in the Sonata and K5 hybrids has 63 percent higher power density and 13 percent higher energy density than nickel-hydrogen batteries used in other hybrid cars available on the market.

Power density refers to the efficiency of power transfer, while energy density indicates the amount of energy that can be stored within a given volume of battery.

In addition, the lithium ion polymer battery pack is 25 percent lighter than conventional batteries used in hybrid electric vehicles, allowing higher fuel efficiency.

The Hyundai Sonata and Kia K5 hybrid vehicles are fitted with six-speed automatic transmission developed specifically for hybrid electric vehicles.

Along with such features, Hyundai and Kia have given their first gasoline-electric hybrid cars the latest interior features including a 4.2-inch color TFT-LCD supervision cluster.

The Korean carmakers have rolled out compact sedan Avante and Forte hybrids, but demand has not been so strong.

In the U.S., Hyundai Motor has the goal of overtaking Toyota Motor in sales of hybrid sedans. The Sonata hybrid has ranked second following the Japanese carmaker’s Prius hybrid.

The sales gap is still wide. The Sonata’s sales reached 1,422 units in June compared to 4,340 units for the Prius, which has already dominated the global hybrid vehicle market.

But what is noteworthy is that the newcomer Sonata hybrid outpaced other hybrid models such as Toyota’s Camry, Honda’s Insight, Honda’s CR-Z and Ford Motor’s Fusion.

Company spokespeople say that Hyundai and Kia have been focused on promotion of “100-percent independent” technologies of the affiliated automakers applied to development of the hybrid Sonata and K5.

By Kim Yon-se (kys@heraldcorp.com)