The Tennessee Valley Authority (TVA) has bought 13 hybrid gas- and battery-powered vehicles in order to boost its fleet mileage by 20 percent over the next five years, according to the Chattanooga Times/Free Press. TVA has bought three Toyota Prius hybrids and 10 Honda Civic hybrids over the past two years to use in Chattanooga, TN, and seven other TVA locations.

TVA officials say the cars reduce gasoline consumption and can boost mileage up to 52 miles per gallon using a combination of electric power stored in vehicle batteries, without requiring electrical hookups and frequent recharging of car batteries. The hybrid vehicles recharge themselves with an electrical motor that supplies power during acceleration and recharges the batteries during braking. The hybrid vehicles can travel 634 miles in the city and 620 miles on the highways on a single tank of gasoline, reported the Times.

Although they comprise a small fraction of TVA’s entire vehicle fleet, more could be added in the future, Bill Lehman, manager of TVA’s vehicle fleet, told the Times. Lehman added that as the reliability of the vehicles increases, prices decrease. However, TVA officials want to test the hybrid vehicles to make better cost-comparisons with conventional vehicles.