One way to look at it is to consider how much mass at what velocity one is hucking out the back. Again, 100 tonne rocket, 2 g acceleration, V exhaust is 5,000 km/s. Yes, mixing units is Satan's work. MV = mv, where M is ship mass, V ship velocity, m reaction mass, v reaction mass velocity.
We want MV/v = m. If we want to add 20 m/s in one direction, then (100,000 kg x 20 m)/5,000,000 m/s = 0.4 kg get thrown out the back. Ek = 1/2mv^2, so .2 x (5,000,000 x 5,000,000) ... 5 trillion watts? I think. My coffee is broken and my calculation is downstairs.
yeah over the years i’ve learned that “physics calculations on my phone” tend to go awry.
This ignores mass ratios, which is OK because so do most SF novels.
I've got a product that gives authors easy lookup tables for mass ratios...$3! Cheaper than a cup of coffee! Or a bottle of dihydrogen monoxide at an airport or concert venue!
Oh, I can do mass ratios. It's just that in this case, it's an unwanted complication.
Heinlein had a odd quirk, which is that he could do mass ratios as long the power source was moderately shitty (chemical, single-H, NTR) but he seemed to think they didn't apply to total conversion drives.
This is actually an area where slide rules would probably be pretty useful.
My slide rule is upstairs. On average, my computing devices are where I need them.