Assuming that’s it’s just the normal force of Earth’s gravity at work on the witch, it’s just a case of working Newton’s second backwards with the gravitational “constant” acceleration ≈ 9.8 m/s²:
F = ma
980 N = m × 9.8 m/s²
980 / 9.8 = (m × 9.8) / 9.8, units omitted for text clarity
100 kg = m
In other words, op is assuming that an average size witch masses approximately 100kg (or about 220 lbs in fingers, knees, and toes units).
Taking a bit of a shot in the dark as I haven’t made it off of Nauvis yet, but what happens to the ingredients/ eggs when the assembler is unpowered? I don’t know how it plays into the spoilage mechanics, but theoretically you could isolate the power grid with a switch and throttle production that way.