Power plants appear to have all stats (power output, build cost, research cost) linked with the size in a linear fashion.
The only difference I see between small and big power plants is thus the rounding of hits-to-kill (HTK) which is linked to hull size (HS = 50t).
- with HS of 0.1-0.9 you get 0 HTK
- with HS of 1 you get 1 HTK
- with bigger HS you get at best (0.5*HS) HTK (2 HTK at HS of 2, 3 HTK at HS of 4...)
With all other stats linear, why to ever use power plants bigger with which you get 1 HTK per 1 HS?
You can get the same output from 1x HS4 power plant as from 4x HS1 ones but with multiple smaller ones you get more HTK, with lower research cost and lower repair cost.