If you were wondering how electric bus manufacturer Proterra managed to secure a large $140 million round of financing last week, it might be easier to understand now that we know they had a massive order in the pipeline.

King County’s Metro Transit, which includes Seattle’s metro area, announced a zero-emission transition of its fleet of buses and confirmed that it will add 120 new all-electric buses over the next 3 years. Proterra is getting the bulk of the order.

While Metro will acquire up to nine long-range electric buses from different manufacturers in order to test them before completing all the orders for the 120 buses, the agency already committed to buying “up to 73 all-electric battery buses from Proterra at a cost of up to $55 million.”

To get some perspective, that’s more than 20 percent of Proterra’s entire sales since its inception.

The first 20 of the 73 are already confirmed. 8 will be in service this year and 12 more by 2019.

County Executive Dow Constantine highlighted the fact that the county was already ahead in term of green public transport with the nation’s largest diesel-hybrid bus fleet, but it is now going all-electric:

“From our electric trolley bus fleet to building the nation’s largest diesel-hybrid bus fleet, King County has long been an innovator in clean vehicle technology. Now, we’re dramatically expanding our zero-emission electric buses and working with the industry to innovate and offer next generation vehicles that move people quietly and cleanly while helping meet our climate goals.”

Most of the buses are 40-foot, but the county is asking the industry to start working on 60-foot all-electric buses in order to replace the articulated buses that make up 55 percent of its fleet.

Metro Transit General Manager Rob Gannon said about the announcement:

“To better serve our customers, we want battery buses that travel longer distances and can carry more people. We’re committed to expanding our battery bus fleet, and need the industry to accelerate development of standardized battery bus charging systems so they can work flexibly for any bus route, and also build more 60-foot-long articulated buses – which serve as the transit workhorses in King County.”

Proterra is working on longer range buses – more specifically its Catalyst E2 all-electric bus with 350 miles of range on massive 660 kWh battery unveiled last year.

The company recently hired Tesla’s former Vice President of Manufacturing to lead a production expansion at their facility in Greenville, South Carolina, and their new factory in Los Angeles County in order to satisfy the increasing demand from transit agencies like King County’s.