How heavy is power seat compared to manually adjusted seat?

For example, in MB C class, power seat is optional. I guess the suspension system is the same no matter whether one chooses this option or not. So a C class with power seat will have less load capacity. How much less?

Depends on how many functions are provided by the power seat.

On the SL for example, I can spot no fewer than six motors and associated hardware (with the cushions removed). Each motor looks to weigh about 2 pounds, and there is the added weight of the electronic module and harnesses mounted underneath the seat. I would say maybe 20 pounds extra?

Too negligible to be factored into suspension stiffness...after all, the occupant weight has more of an impact (120 vs. 220), and they engineer that into their parameters...

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2009 ML350 (106K) - Family vehicle
2001 CLK430 Cabriolet (80K) - Wife's car
2005 BMW 645CI (138K) - My daily driver
2016 Mustang (32K) - Daughter's car

I just had a leather set shipped to me and both front seats weighed in at 75lbs each.

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1960 MB 190Db w/ full sunroof
1985 Toyota Pickup
2006 Honda Odyssey EXL R&N
2001 VW Jetta GLS TDI