A dehumidifier's headline "12 litres a day" is measured in a warm, very humid test room. A UK living room is cooler and drier, and the same machine removes less water there.
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Size my dehumidifierThe test conditions matter more than the number
Capacity is the water a unit removes in 24 hours under set conditions. Warm, humid air carries far more water than cool, drier air, so a test in hot, humid air flatters the result. At 30 °C and 80% relative humidity the air holds roughly 24 g of water per cubic metre. At 20 °C and 60% it holds roughly 10 g. These are our own sums from standard saturation values, and they show why the same machine cannot match its headline figure at 20 °C.
What the figures look like for models we list
Where a maker publishes output at both conditions, the gap is large:
| Model | Headline rating | Maker's figure at 20 °C, 60% RH | Difference |
|---|---|---|---|
| Meaco Arete One 12L | 12 L/day at 30 °C, 80% | 5.23 L/day | About 56% lower |
| Meaco 20L Low Energy | 20.38 L/day at 30 °C, 80% | 8.48 L/day | About 58% lower |
The Meaco DD8L Zambezi (desiccant) is rated at 20 °C and 60% RH to begin with, so its 8.3 L/day needs no adjustment. That makes it a fairer like-for-like with other models' 20 °C figures than with another model's 30 °C headline. Details are on each product page. For the other models we list, the maker has not published the 20 °C figure, and for some not even the test conditions, so we show those cells as blank.
This is not just a UK habit
In the United States the test has moved the same way. The Department of Energy's 2020 rules for room dehumidifiers test at 65 °F (about 18 °C) and 60% RH instead of the earlier 80 °F (about 27 °C). An Energy Vanguard article gives an example of a unit rated 30 pints a day under the old test and about 20 under the new one, a drop of around a third with no change to the hardware. A cooler test gives a lower number, which is closer to a typical home.
Why makers use the hot test
It produces the bigger number, and buyers compare the biggest number. It is also a long-established convention. It is not dishonest as long as the conditions are stated, but it makes models hard to compare unless every figure uses the same conditions. We cannot tell you which conditions a maker used unless it says.
How to use this when you shop
- Look for the conditions. A rating without a stated temperature and humidity cannot be compared with one that has them.
- Prefer a 20 °C, 60% figure if the maker gives one. It is closer to a heated UK room.
- Treat the headline as a best case. In a cold room expect much less, and consider desiccant or hybrid types, which hold up better in the cold. See compressor vs desiccant.
- If in doubt, go up a size. Compare running costs using our cost method so a bigger unit does not surprise you.
How we handle it
Our product pages show the rated figure, the conditions where known, and the 20 °C, 60% output where the maker publishes it. Our sizer works from headline ratings today, so read its capacity bands as a best case and favour the upper end of a band for cold or damp rooms. See how we choose. We have not tested these machines ourselves.
Sources: Meaco product specification tables, as recorded on each product page; Energy Vanguard, The confusing world of dehumidifier capacity; ENERGY STAR, Dehumidifier testing and capacity. Percentage differences and water-per-cubic-metre values are our calculations.