Ok, I haven't read through all replies (to lazy to read every one) BUT continueing this strong rainforest analogy...possibly a slightly alternative system
Both of you are correct in, overcrowding leads to light loss but the veggies dont cover entire screen...Just have NRG distributed equally across the sim, then have it based off size...because if a plant had super-huge leaves, it'd get more energy from the sun...(Though irl that would also mean more water loss...but were susposing these sims are in swamp-like conditions, like all non-biome-specific sims) and then we have a nice simple system, more area you cover = more energy you get.
(Just work it off a tiny grid and each box your in, you get that boxes energy.)
of course this means big-bertha veggies get ridiculously huge, so you'd need to work on bot enviroments (see my pollpost) so you'd need a huge-veggie disadvantage...but yep, that'd be a easiest system
