Where the thousand Btu comes from
Evaporating water absorbs energy without changing its temperature, and that energy is the latent heat of vaporisation. Near room temperature it is somewhere around a thousand Btu for every pound. That is the number doing all the work on this page, and it is a field rather than a constant because it moves with temperature — a psychrometric table gives the figure at the condition your room actually runs at, which is why the hint sends you there.
The chain is short. Gallons times 8.3454 gives pounds. Pounds less whatever stays in the plant tissue gives the pounds that evaporate. Divide by the hours it happens over to get pounds an hour, multiply by the latent heat, and that is Btu an hour of latent load. Forty-five gallons a day at 5 percent retained, over 16 hours, at 1,050 Btu/lb, is 23,415 Btu an hour.
Why sensible and latent must not simply be added
The tempting mistake is to take the electrical input, convert it to Btu, and add the latent load on top. That double counts, because the energy that evaporated the water came from the fixtures. In a sealed room the whole electrical input ends up as heat, and the latent load is the share of it that went into vapour rather than into raising the air temperature. So sensible is electrical input minus latent, not electrical input plus anything.
With the defaults, 10,500 W is 35,827 Btu an hour going in. The latent share is 23,415, which leaves 12,412 as sensible. A sensible heat ratio of 0.35 is the number worth staring at: two thirds of the duty in this room is moisture.
Tonnage is the wrong unit to shop in
Cooling equipment is sold on a tonnage that describes total capacity at a rating condition, and that condition assumes a sensible heat ratio far higher than a grow room has. A unit sized on total Btu will hold the temperature setpoint comfortably and leave the humidity somewhere the grower did not choose, because it never runs long enough to condense what the crop is producing. That is the reason dedicated dehumidification exists in these rooms rather than being an optional extra.
The pint figure on the page is the same water expressed the way dehumidifiers are rated. Three hundred and fifty-odd pounds of vapour a day is close to 342 pints, which is two 200 pint machines at their rated figure — and rated figures are stated at a test condition that is warmer and wetter than most grow rooms, so treat the count as the beginning of the conversation.
The condensate is not nothing
Every pound the coils take out arrives in a pan. Forty-three gallons a day is water that was already paid for once, and in a room where make-up water is metered it is the single largest recovery available. Whether it goes back into the system is a water quality question rather than an arithmetic one, and this page only says how much of it there is.
Questions people ask
What latent heat figure should I use?
The one from a psychrometric table at the temperature your room runs at. It falls as temperature rises, and the difference across a normal room range is a few percent rather than a few times, so it is not the input the answer is most sensitive to. The water uptake figure is. The field defaults to 1,050 as a placeholder to replace, not as a fact.
How do I measure the water uptake?
Irrigation delivered minus what came back to the drain, over a full day, with a meter on both. On a recirculating system the change in the reservoir level over a day works if nothing else was added. On a small bench a scale under the tray is the cleanest reading of all, because it measures exactly the water that left.
Why does the page tell me the sensible load is zero sometimes?
Because the latent load came out larger than the electrical input, which cannot happen in a sealed room. Rather than print a negative number it flags that two inputs disagree. Usually the uptake figure includes water that went to drain rather than into the air, or heat is arriving through the envelope from a warm building around the room.
Does this include heat coming through the walls?
No. It is a balance of what happens inside the insulated envelope: electrical input in, latent and sensible out. Conduction through walls and ceiling, air leakage and any deliberate fresh air are separate loads, and in a room built inside a warm building they can be large. Add them from an envelope calculation rather than assuming they cancel.
Can I use the condensate as irrigation water?
The page tells you how many gallons there are and stops there. Whether condensate goes back into a nutrient system depends on what it picked up from the coils and the pan and on how the system is managed, which is a water quality question for whoever handles yours. The number is worth knowing either way, because it is the largest single recovery in most of these rooms.