But there are different lights in veg and the flower room.
And then there is the fact that every plant is different each time. So many variables to consider.
So for each plant you take the hours spent in the room and multiply the watts of the light by how many hours its on. Say in veg 400w 18hrd a day for 3 weeks. So 400x18x21=151,200 converted to kw hrs. 151.2 kw hrs.
Then say 600w flower. For 8 weeks. 600x12x56= 403,200 converted 404 2 kw hrs
Then add them. 403.2+151.2= 551.4 kw hrs.
Say yield is 600 grams. That a little over 1 gpw.
Easier to add up all the lights on a monthly basis. Like 18hrs x 30 days x 400w. Do the same for each room and add them all together and you knkw the total kw hr of light for the every month. Each month just add the yields and calculate.
Hell you could do it on a yearly to get more accurate.
Its easy to do you just have to figure out in which way you want to calculate it.