These vehicles will join two other Optare MetroCity vehicles currently in service on that route (312, between South Croydon and Norwood Junction) later this year.
"Following an initial trail of the Optare fully-electric vehicles on route 312 we were satisfied that the technology was robust enough to stand up to the rigours of operating in an intense urban environment," states Peter Batty, commercial director at Arriva London.
"This is an exciting new milestone for our bus fleet, which is already one of the cleanest in Europe," comments Mike Weston, TfL's director of Buses.
"We currently have eight electric buses in the TfL fleet and this number will grow to more than 300 by 2020," he adds.
And Enrico Vassallo, CEO of Optare, says: "This marks the first order of electric vehicles to be solely funded by a London operator.
"With zero emissions at tail pipe, these vehicles demonstrate that fully-electric bus technology has very much arrived and is a commercially viable solution to improved air quality in London."