Efficacy is the exchange rate
A watt is a joule a second. An efficacy in micromoles per joule turns that stream of joules straight into a stream of photons, so watts times efficacy is micromoles per second and nothing else is needed. Nine thousand watts of fixtures at 2.7 umol/J is 24,300 umol/s leaving the fixtures. Take 15 percent for what lands on walls and aisles and 20,655 umol/s reaches the canopy.
Spread that over 768 sq ft — 71.35 square metres — and it is 290 umol/m2/s on average. Run it 16 hours and the daily integral is 290 times 57,600 seconds, divided by a million because a mole is a million micromoles, which is 16.7 mol/m2/day. The million is where unit errors live, and a factor of a million is not the kind of mistake that looks wrong on the page.
Why the derating is a field and not a constant
Lighting on a photoperiod is a continuous load by any reasonable reading, and every electrical code applies a derating to continuous loads on a branch circuit. The figure is not this page to state. Codes are adopted jurisdiction by jurisdiction, amended locally, and enforced by an authority that has the last word. So the derating is an input, with whatever number your electrician or your inspector gives you in it, and the page multiplies by it without opinion.
The same goes for whether a 50 A feed is a 50 A feed. Conductor sizing, ambient temperature, bundling, the panel, the terminations and the difference between a breaker rating and a usable rating are all real and none of them are here.
The three ways out of a shortfall
When the feed does not reach the light level you wrote down, the arithmetic offers exactly three exits and the page prints all three. Light less canopy at the level you want. Run longer hours at the level the feed gives. Or get more electricity into the building.
They are not equivalent. Shrinking the canopy costs production directly. Longer hours costs energy, moves the peak on the meter, and runs into the fact that some crops need a dark period — a species question the page has no view on. A new feed is a capital project with a lead time measured in months. Knowing which one you are choosing is more useful than a single answer.
What an average PPFD hides
Dividing total photons by total area gives a number no point in the room actually reads. Real footprints peak under the fixture and fall off at the tray ends, and on a tiered rack the fall-off at the ends of an 8 ft tray is where the uneven crop shows up. The average is the right number for sizing a feed and the wrong number for judging a tray. A grid of meter readings at the height you actually mount at is the only thing that settles the second question.
Questions people ask
Where do I find the efficacy of my fixture?
On the photometric report, which is where photosynthetic photon flux and input power are both stated. Divide the flux in micromoles per second by the input watts. If only a flux figure is published and no input power, or only a wattage and no flux, the number cannot be derived and no substitute figure should be assumed.
Does this tell me whether my breaker is big enough?
No. It multiplies the rating you typed by the derating you typed, which is arithmetic rather than an assessment. Conductor sizing, ambient conditions, the panel, the terminations and what the adopted code actually requires are all outside this page and belong to a licensed electrician and to the authority that enforces the code where the building is.
Why is the capture share so important?
Because it sits directly in the multiplication and nothing else corrects for it. At 85 percent capture, 24,300 umol/s of output becomes 20,655 on the canopy. At 70 percent it becomes 17,010, and the PPFD falls by the same proportion. On a tiered rack with tray ends and aisle gaps the honest figure is often lower than people assume, and it is measurable with a meter.
Can I use this backwards to size fixtures?
Partly. If you know the canopy and the DLI you are working to, the page gives the PPFD that needs and the area this feed reaches it over. Turning that into a fixture count needs the individual fixture flux and its footprint, which are on the same photometric report. This page deliberately works from the feed rather than from a fixture, because that is the constraint most rooms actually hit.
Does a three phase feed give more?
It gives volts times amps times the square root of three per feed rather than volts times amps, which for the same line to line voltage and the same breaker rating is about 73 percent more capacity. Selecting three phase applies that factor. Whether a three phase supply exists at the building, and how the lighting is balanced across the phases, are separate questions this page does not touch.