Would 6 150w fixtures produces less heat than 1 900watt fixture?

Compare efficacy and it's not a guess. The size of the fixture is not the cause. If it's less efficacious (gonna use 'efficient' even though it's not quite right from here out), it produces more heat per kw-h expended. you may not notice it as much at 150w aas 900w, but it's the same rate of heat being produced if same efficacy. Unfortunately, manufacturers of grow ligths lie through their teeth abuot efficacy and general light-related specifications quite often. Mars, spiderfarmer et al all exaggerate or use the best diode at testing specs but not representatitve how it is used in the light or the fact other diodes are invovled. 5000K is more efficient than 3000K, with all other factors being the same. Those 660nm far red chips are significantly less efficient more times than not. I forget the brand, but there is a 2.9umol/J red chip but they never use it due to expense. they use osram more often. So, compare efficacy - it is in the form of umol/J (micromoles per joule). 1 w/s = 1 j/s conversion, so that part can be assumed without any math. The per second cancels out. If it's 2.8umol/J that means 2.8micomoles/s of PAR generated per 1 joule/s of heat. Anything listed in that upper ranage of 2.8-3.0 should be verified. Anything over 3.0 is probably a lie unless it is purely a 6500K diode and nothing else.. also best quality they sell of the 6500k and running at 0.2 watts per diode. other retarded things you will read on a light spec sheet - "PPFD at a certain distance" - absolutely not how ppfd is calculated or used, bwahaha. Does not make sense. The most efficient light will produce the least amount of heat... doesn't matter if it's 4 lights or 1. I guess you might have some potential variance due to different drivers, but those are all a similiar efficacy at max load. The biggest differences would be the ones that handle

Once you start getting Into high end lighting you may actually get less lumen per watt on higher quality led. This is because they generally use different tech to produce the reds high phosphors need more energy to produce the elusive high quality reds above 90+ cri. Cheapy led light provide 80 lumens per watt. High end led provides closer to 60 lumen per watt. Not sure what's more beneficial overall.

Firstly It depends on efficiency of the led, how much electric energy gets converted to light and how much of that gets converted to heat as a by product. The more efficent a light is the less energy is lost through heat. Secondly and often overlooked is heat sinks. They are very costly given they are chunks of copper or steel and often cheap lights do not come with much in the way of heat sinks. By increasing the amount of surface area you can cool more efficiently. 900w is still 900w regardless if it's 9x100w or 1x900w. The efficiency of the conversion and the amount of surface area for dissipation will effect how well the light will release that heat.

Simply put, NO. That is not how the wattage unit would be understood and used for the comparison. Math and science would say the delta would be negligible. I'm sure you could exploit some geometry and more optimized fixtures by using small ones, but if you're comparing their wattage draw then the only difference is going to be ballast effeciency, diode effeciency, and surface area for heat sinks. Aka your loss. I can say from experience that you better get some meters if you plan on doing this, your PAR map is gonna be wildly off the manufacturers. Otherwise, I prefer more smaller fixtures rather than a large one.

I would say it would be hotter with more lights even if small as it's more hardware running in the end, heat transfer from the diodes will be determined by manufacturer.