I've got many ideas, but the most straight one would be to drop sunlight metabolysm to zero whenever a gene is executed.
Rationale behind: veggies need to do 3 steps metabolism, to convert light into energy molecules, then use those to build sugar, proteins and fat, then burn/use/store them; while animals take sugar, proteins and fat directly, and then burn/use/store them.
So, the action to gather energy from light would take a full cycle on sun gathering metabolism (idle).
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Later, a curve could be added to the light metabolism -as, time needed to set up in a good sun bathing position and synthetize full chloroplasts- so the first idle cycle it gets 20% of the full energy, the 2nd, 40%; the 3rd, 60%, till the 5th and forthcoming when it will receive 100% of the selected energy per cycle.
A veggie that idles 1 of each 2 cycles on a sim with 100 energy per cycle would make
20 + (exec) = 10 energy average
A veggie that idles 4 of each 5 cycles on a sim with 100 energy per cycle would make
20 + 40 + 60 + 80 + (exec) = 40 energy average.
A veggie that idles 9 of each 10 cycles on a sim with 100 energy per cycle would make
20 + 40 + 60 + 80 + 100 + 100 + 100 + 100 + 100 + (exec) = 70 energy average.
A veggie that idles 99 of each 100 cycles on a sim with 100 energy per cycle would make
20 + 40 + 60 + 80 + 100*95 + (exec) = 97 energy average.