Delivered via a Chevy Volt plug-in hybrid, Gray's letter said, "We stand ready to defend our title . . . from any city - despite its size and resources." He closed in capitals: "BRING ... IT ... ON!"
Mayor Nutter could not attend the event, but he sent a video statement. "Watch out, D.C.," he said. "When our residents, businesses, and organizations join together, I have no doubt we will become the greenest city in America."
Listening to the chatter was Swarthmore Borough Manager Jane Billings, who noted with a smile that total kilowatt hours purchased is only one way to measure green-power usage.
There's also the percentage of power that's green, and in that department, tiny Swarthmore has both big cities beat.
The Delaware County borough now ranks fourth on the national list of municipalities, with 27.9 percent of the energy used inside the borough's limits coming from green sources.
But Billings declined to join the trash talk, no matter how good-natured. To her, it's all good, and "I'm thrilled to see that a city as big as Philadelphia could come this far."
Perhaps it's no surprise that a town like Swarthmore would go strong for green power. But a mammoth, complex city like Philadelphia?
A tough job, sure, said EPA regional administrator Shawn M. Garvin. "But the beauty of it is that you have Swarthmore, you have Philadelphia, you have other cities, small, medium and large. The more examples we have, the better."
A municipality can join the program by meeting minimum purchase requirements for green power - ranging from 3 percent to 20 percent depending on power usage. It then agrees to start a campaign to encourage businesses and residents to buy green power.
And the more green power purchased, the more the impact on carbon dioxide levels, climate change, and overall air quality in the city, Garvin said. "It also creates green jobs. And it helps to save money."
As an EPA Green Power Community Partner, Philadelphia must have 4.1 percent of electricity citywide come from green sources - a goal that's been met. Philadelphia's residents, businesses, and institutions buy about 12 percent green energy, according to the city's Greenworks plan.
It's a help that several large institutions purchase 100 percent green energy, including Drexel University, the Academy of Natural Sciences, and Yards Brewing Co.
Yards president Tom Kehoe said the decision to go with green power - to keep Philadelphia Pale Ale fermenting at 68 degrees at its Delaware Avenue brewery - was made at the instigation of its younger employees.
It costs a tad more, Kehoe said, "but with everything we've gotten out of it, including a lot of self-pride, it's definitely worth doing."
Other major purchasers include the Eagles and the University of Pennsylvania. Nutter said at least 400 smaller businesses also buy green power.
The city itself has secured wind power credits to cover 20 percent of municipal government's electricity consumption for 2012, which is costing about $37,000 in general fund money.
City energy manager Kristin Sullivan said the city was still paying less for electricity overall than if it had stayed with Peco Energy after rate caps were removed in early 2011. The city also gets green power from a solar farm at a sewage treatment plant.
Meanwhile, green-power producers are gearing up campaigns and special offers - with competitive rates, they say - to attract what they hope is a gusher of new customers. The state Public Utility Commission's green-power website, www.PaPowerSwitch.com, lists about 15 suppliers that offer green options in Philadelphia.
On Wednesday, one company, Green Mountain, pledged to offset LOVE Park's electric load for the day by buying solar renewable energy credits, and was giving away water ice made with solar power. Water in the park's fountain was tinted green.
Rina Cutler, deputy mayor for transportation and public utilities, said Washington's mayor "may not know how competitive we truly are."
She said she only hoped the Volt that carried his letter north "will fuel up at one of our 20 recently installed electric-vehicle charging stations before it begins its trip back."
"Plus, we expect to beat the Nats as well."
Contact Sandy Bauers at 215-854-5147, email@example.com, or follow on Twitter @sbauers. Read her blog, "GreenSpace," at www.philly.com/greenspace