It's photons and photons are all the same.
Sunlight has a different spectrum than an LED and, since it's through a window, it's not going to be a lot of light. One issue to watch out for is heat from the sunlight because sunlight has IR in its spectrum and if it's behind glass and not getting some ventilation, the leaves might get too warm.
Re. too much light - some growers give their plants too much light. It's not common but I have seen a few instances of that in 3 ½ years here on RIU and on another site. I've done it myself, at least twice, but I push my plants pretty hard, with light levels over 1100µmol in some cases. I all cases, once you turn down the level level, the plant uncurled its leaves and returned to normal but I did have one cola that bent a little and it didn't straighten up. Having said that, the light levels my plants were getting we well beyond where, until maybe a couple of year ago, what most lights could generate.
My observation is that growers lose far more yield by not turning up their lights than they do by giving them too much light. That's not a big stretch because it's almost impossible to damage a plant by giving it too much light. In contrast, cannabis looks just great when it gets, say, 50% of its light capacity but growers, by and large, aren't aware of that.
"It looks really nice and I'm getting 2-4 ounces from a plant so everything's cool right?"
"Sure but if you run your light at 100% you will double your yield."
That's an extreme case but not a rare one.
"Yeah, I'd read that too much can cause light burn & stress in photo-period plants but couldn't find the same answer for autos."
I've grown photos at >1100µmol. I've run my autos at >1000µmol with DLI's in the 80's at times. I stopped growing autos because they're hard to tame, With photos, you can control plant size because you control the length of the vegetative stage of growth. Autos work on they own timetable so they can get pretty mangy.
One of these an auto, the other is a photo.
This nice little plant looks like an auto but it's a photo that was grown under a veg light (makes plants short and compact with lots of leaves), topped, and LST'd.
There are two plants in here - if you look really closely, you can see Mary tucked away on the left side. These were Gelato autos from the same seed packet. Jeff was a monster. I turned the light down to 800µmol about ⅔ way through flower because there was so much weed in the tent there was no sense in running my light any higher.
Crop yield increases in an almost linear manner as light levels increase, up to the light saturation point, the light level where a plant can't process any more photons.
This chart is from the cited paper, showing the increase in yield as light levels increase. Roughly speaking, yield increased 4% for every increase in average PPFD of 50µmol. The curve does drop off but, pick a couple of light levels and see how much more weed was produced.
Or, better yet, go from high light levels to low light levels and see how much weed wasn't produced because the grower didn't turn up the dimmer.
