Mass.-based Worcester Regional Transit Authority (WRTA) purchased three additional zero-emission all-electric transit buses from Proterra Inc.

The purchase increases the WRTA’s electric fleet to six buses — 13% of its entire fleet. The buses are set to go into service this month, giving WRTA the largest fleet of electric buses in revenue service in North America.

The WRTA made its initial purchase from Proterra in September 2012, using funds from a $4.4 million Federal Transit Authority Clean Fuels grant to replace three of its 12 old diesel buses. The three electric buses went into service in late summer.

“Repeat customers represent the ultimate customer service goal for our company,” said Garrett Mikita, Proterra’s president/CEO. “Our continuing partnership with WRTA emphasizes the long-term financial benefits of reducing an authority’s dependence on fossil fuels and the positive impact battery-electric buses can have on the environment.”

Proterra’s EcoRide BE35 battery-electric buses recharge in less than 10 minutes. The buses are made of lightweight composite materials and are powered with state-of-the-art battery technology. As a result, the buses have zero emissions, run virtually silent and are 500% more fuel efficient than diesel buses.