For all that she controlled the first few laps from the front, her closest pursuers, Powell and Finland's Emma Kimilainen, swept past her with unexpected ease, soon finding themselves nine seconds up the road.
It meant that Chadwick, who had started the race from pole before being passed by Powell, Kimilainen and Visser, won the title by 10 points from Visser.
That put Visser on Chadwick's tail and the Dutchwoman overtook her with three minutes left after a breathtaking passage of wheel-to-wheel action during which the pair made contact. "The pressure just got to me a little bit".
Chadwick's only title rival, Beitske Visser, qualified fifth and ideally needed to scoop victory to have a real chance of denying the British driver title glory. As Powell led the field into Clearways and the safety auto peeled into the pit lane with six minutes remaining in the 30-minute race, she cleverly bunched up the field then nailed it onto the straight, establishing a lead over Kimilainen that she would never lose.
FIA Superlicense points - 40 of which are required to race in F1 - will be up for grabs next season as W Series expands, but Chadwick revealed she may not defend her title in 2020. "Whether that's personal or through sponsors or a team, none of them have paid their way scraping the barrel", she said.
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Chadwick pockets a prize of £410,000 and, having been signed as a development driver for Williams, she keeps up her hopes of making it into Formula One.
"To have the championship crown today is pretty wonderful".
"I want to do as much as I can.my overall aspirations are much greater than just this, than the W Series", she said. Next year, the W Series victor will also get points.
"I think we've done that now and next year has got to be all about the drivers and telling the drivers' stories and introducing the personalities and characters".
Chadwick felt the reality, with the series always stating it wanted to prepare women to take on the men in junior series such as Formula Three and Formula Two, had changed some preconceptions. Powell was third overall on 76.