The defeat means that the Welsh county's chances in the FP Trophy this season are realistically over.
Powell's chanceless 140-ball innings at the Swalec Stadium - which contained eight fours and a six - was Glamorgan's first century against first-class opposition in a one-day game since 2004.
It followed the Abergavenny man's landmark of 10,000 first-class runs, achieved during the Championship game with Derbyshire on Friday.
"I've been feeling happy with my game this year but haven't been kicking on and getting the big scores, so it's good to change that," Powell told BBC Sport Wales.
Glamorgan now face two games in Swansea, Wednesday's Championship match with Northants followed by a FP Trophy clash with Worcestershire on Sunday.
The game will be difficult for Powell, who picked up an injury at St Helen's last year that left him with a blood clot in his arm.
Complications meant the problem became life threatening, the batsman undergoing four operations and missing the rest of the season.
"It will be a strange one going back to Swansea as that's where it all went wrong for me last year," said Glamorgan's 2006 player of the season.
"But I won't have any superstitions, I always enjoy playing at St Helen's.
"Confidence is high in the Championship, we're playing some good cricket."
Glamorgan have added Ryan Watkins and Alex Wharf to their squad for the Northants game, Alex Wharf dropping out.
Jason Gillespie is an injury doubt, though, and faces a fitness test on his hamstring injury on Tuesday.
Glamorgan (from): Gareth Rees, Matthew Wood, David Hemp (capt), Michael Powell, Jamie Dalrymple, Mark Wallace (wk), Robert Croft, Jason Gillespie, James Harris, Dean Cosker, David Harrison, Ryan Watkins.
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