What size dehumidifier do I need?
Plants breathe out almost all the water you give them. So your dehumidifier has to pull out about what you water each day, minus what drains away, plus a margin. Then check the label: since 2019, pint ratings are smaller for the same machine.
Your result
How it works, in plain words
Water in, water out
About 97% of the water a plant takes up is transpired. Four flowering plants at 2 L a day with 10% runoff put about 7 L (15 pints) into the air every day, in line with the 3–6 L per m² a day that canopies typically use.
Rating
Buy at least that × 1.25, on the 2019 DOE label. Old AHAM labels (tested at 80 °F) read about 1.4× higher for the same machine: a former 70-pint unit is now a 50.
Lights-off spike
Cooler air holds less water. 55% RH at 79 °F becomes about 70% at 72 °F with the same water in it. Keep the night drop small and let the dehumidifier run in the dark.
Heat
A dehumidifier is a heater too: its watts × 3.412 go into your cooling load.
Every number this calculator uses
The same constants feed the Calculate it properly guides, so the tools and the articles agree. Rules of thumb are labelled as such.
| Constant | Value | What it is | Evidence |
|---|---|---|---|
TRANSPIRED | 0.97 fraction | Share of the water a plant takes up that it transpires into the air | Established |
NIGHT_RATE | 0.1 × day rate | Night transpiration per hour as a share of the lights-on rate | Supported (other crops) |
DEHU_MARGIN | 1.25 × | Dehumidifier margin over the average daily water (the AC also pulls water while it runs) | Grower practice |
PINT_PER_L | 2.11338 US pints per L | Dehumidifier ratings are in US pints/day | Established |
AHAM_TO_DOE | 0.7 × | Old AHAM pints/day (80 °F, 60% RH) to the 2019 DOE rating (65 °F, 60% RH): about 70 → 50 | Established |
DEHU_IEF_S | 1.3 L per kWh | Federal minimum efficiency, portable units up to 25 pints/day (2019 rating); used to estimate the heat when you don't know the watts | Established (regulation) |
DEHU_IEF_M | 1.6 L per kWh | Federal minimum, portable units 25–50 pints/day | Established (regulation) |
DEHU_IEF_L | 2.8 L per kWh | Federal minimum, portable units over 50 pints/day | Established (regulation) |
WATER_SEEDLING | 0.25 L per plant per day | Stage helper: seedling | Grower practice (varies with pot, light and VPD) |
WATER_VEG | 1 L per plant per day | Stage helper: veg | Grower practice |
WATER_FLOWER | 2 L per plant per day | Stage helper: flower (4 plants in a 4 × 4 ft tent ≈ 5.4 L/m²/day, inside the guides' 3–6 L/m²/day) | Grower practice |
Sources
- Taiz L, Zeiger E, Møller IM, Murphy A. 2015. Plant Physiology and Development, 6th ed., Ch. 4 (about 97% of the water a plant absorbs is lost by transpiration). Sinauer. https://global.oup.com/academic/product/plant-physiology-and-development-9781605353531
- Caird MA, Richards JH, Donovan LA. 2007. Nighttime stomatal conductance and transpiration in C3 and C4 plants. Plant Physiology 143(1):4–10 (night transpiration commonly 5–15% of daytime, up to ~30%). https://doi.org/10.1104/pp.106.092940
- GenetixBay house position (founder's field experience, decades of indoor rooms): passive intake for most growers; oversize lights and dim down; replace HPS watt for watt unless the LED is top tier. https://genetixbay.com/editorial-policy
- NIST. Guide for the Use of the International System of Units (SP 811), Appendix B: conversion factors (1 W = 3.412 142 BTU/h; 1 ft³ = 0.028 316 8 m³; 1 US gal = 3.785 412 L). https://www.nist.gov/pml/special-publication-811
- US DOE. 10 CFR 430 Subpart B, Appendix X1 (dehumidifier test at 65 °F and 60% RH, required from June 2019; the older AHAM DH-1 rating used 80 °F and 60% RH) and 10 CFR 430.32(v) (minimum integrated energy factor, L/kWh). https://www.ecfr.gov/current/title-10/chapter-II/subchapter-D/part-430/subpart-B/appendix-Appendix%20X1%20to%20Subpart%20B%20of%20Part%20430
- ENERGY STAR. Dehumidifiers: key product criteria and the 2019 capacity re-rating (a unit once sold as 70 pints/day is about 50 pints/day on the new test). https://www.energystar.gov/products/dehumidifiers
- Alduchov OA, Eskridge RE. 1996. Improved Magnus form approximation of saturation vapor pressure. Journal of Applied Meteorology 35(4):601–609. https://doi.org/10.1175/1520-0450(1996)035%3C0601:IMFAOS%3E2.0.CO;2
