There is so much to this. 37.5 or 50 watts whatever it is that seems to be the concensus is for photos with hps. This is going to take awhile. One factor is if you take a 37.5 watts and keep it 2 feet above a plant it will have a lower ppfd than 20 watts at 12” above the plants. (Ppfd being the amount of light at the canopy). Light height has a bigger impact on intensity than the actual wattage or efficiency. Fact. This is why small efficiency gains are basically irrelevant. Lowering a light as little as 1 inch could have a bigger impact than a 5% efficient gain. Something I have been meaning to mention for awhile. Especially when growers over spend to gain such small numbers.
A hps is a 360 degree bulb, light shines up, down side to side hits a reflector and is then diverted down. All of this movement reduces the total number that hits the canopy. A cob being directional has a huge advantage. This allows for a lower wattage. Secondly is efficiency. Hps is no match for a good brand of cobs.
We are at a hard point here in growing. Auto vs photo. It’s not just a different type plant. Autos have much different needs than photos do and this is another fact. The daily light integral is real. This is something I have lived by and basically built a business around. Not just good lights but a bit of knowledge behind them to offer assistance that’s unbiased to photos and autos. Broken down over a 4x4 for example.
37.5x16sq’= 600 watts. This is where it may get complicated.
600 watts for 12 hours a day. (Flower time for photos) = 7200 watts per day.
600 watts for 24 hours a day ( flower time for autos ) = 14,400 watts per day.
600 watts for 24 hours per day.
As you can see the longer hours affects the total amount of light per day. Now this isn’t just a theory. It’s been my life for over 2 years and first caught my attention when I couldn’t stop frying my plants under this “needed wattage for grow good pot” I spent a lot of time reducing the wattage on my grows and the more and more I did the better the plants turned out. Half of 37.5 is 18.75. To give your autos the same amount of light per day as a photo you can cut these numbers in half. This is why I’m a little concerned with the cannon and it’s release in an auto forum. It is above a photo which is like bringing a gun to a knife fight. Lol. I know for a fact it will do well above the photo but a photo has different needs. I’m not trying to knock anyone or anything but I can almost guarantee much different results than the current test will reveal if it’s above an auto. Just based on the hours. Showing off a great big photo is almost irrelevant unless it’s being sold to photo growers. An auto grower will try and recreate the test results and it won’t have the same outcome.
As far as the OP there is really so much to it. I do understand you are growing a photos and this could be a small part of the result. Wattage wattage we need more wattage. That’s what I hear in my head at night when I try to sleep. This is false. You need light. The best way to get more light as mentioned above is more in the delivery of the light than the light itself. For example someone may say 60 watts isn’t enough. It actually could be enough and over the past 2 years I have proven it is. The “could” part relies on the height of the light. 37.5 at 24” or 20 at 12” so it’s not the wattage that matters. Next is the use of the Angelina reflectors. At just about every height My apogee shows a 40% increase in ppfd. This is where it gets hard. I can’t explain all this to every grower asking to buy lights. But I do try to understand the growers that ask me questions. What they are doing and how they are doing it. Auto, photo, space, grower skill, type of nutrients, reflectors, no reflectors. Trying to find the middle ground. That’s just the light. I do understand the light is usually the first to get criticized when a harvest isn’t great. There are a dozens variables that can affect a yield. It could be gnats early on eating the root, aphids destroying the plant, nutrient issues, lockout, toxicity, lights to high, lights to low.
What I suggest growers do when growing photos is to increase the number of cobs. 5-6 cobs in a 4x4, the addition of reflectors and the lights kept lower. This won’t satisfy the made up wattage requirements but it will contour a bit more to the type of plant you are growing. In the meantime keeping a log and photos here will help. No grower can plant a seed under the best light and 2 months later expect pounds of good product. It’s impossible to go into a conversation knowing nothing more than the lights used and the final yield and come up with a conclusion. Hopefully some of this helps, even though it’s more geared towards autos vs photo.
@Black Sail to go on with our last convo, wattage is wattage. It’s equal to 3.41 btu’s ragardless of the source or its efficiency. You can put a thermometer in 2 identical sealed spaces. In space one put a 100 watt toaster over and in the other a 100 watt cob and the temps will be identical. The only constant with wattage is heat. Light-thermal radiation is heat. The heat at the heat sink created by conduction is heat. More efficient lights give off more light, more thermal radiation. Where the savings is takes place is when trying to get a specific amount of light you can use less wattage with more efficient light sources. The lower wattage will equal less heat and an energy savings. You can’t reduce the amount of heat from a watt though.
I really hope I didn’t offend anyone or break any afn laws. Just trying to work some very important facts/opinions out. I also hope it made sense. I’m nodding off trying to type
maybe dude will give me a shout 
