Dehumidifier litres a day ratings explained

What the rating measures

The rating is the maximum water the machine removes in 24 hours of continuous running in a test room held at a fixed temperature and humidity. It is a lab figure designed to compare machines, not a promise of what you will collect at home.

The higher the temperature and humidity in the test, the more water there is to extract and the more efficient a compressor coil is, so the figure goes up. That is why the test condition matters as much as the number itself.

The common test conditions

You will see a few conditions quoted on UK listings and spec sheets. Check the small print on the box or the maker's website.

Why 30 °C and 80 % looks so much bigger

At 30 °C air holds far more water vapour than at 20 °C, and at 80 % RH it is close to saturated. A compressor coil running at a fixed temperature therefore condenses much more water from that air. Drop to 27 °C and 60 % RH and the air holds less water and its dew point is lower, so the same machine collects much less. The gap between the two figures is often large, and some makers show both so buyers can see it.

Neither figure is wrong. They just answer different questions. If one maker quotes 30 °C and 80 % RH and another quotes 27 °C and 60 % RH, the second machine may actually be the more capable of the two despite the smaller number.

What a 20 litre dehumidifier means at home

A 20 litre rating at 30 °C and 80 % RH means the machine can remove up to 20 litres a day in that test. In a heated UK room at around 20 °C and 55 to 65 % RH, it will collect far less, because there is less water in the air and the coil works less efficiently. Once the room reaches your target humidity it also stops extracting.

So treat the rating as a size class rather than a daily yield. A 20 litre model is a larger, more capable machine than a 10 litre one and will reach a target faster, which is why our what size dehumidifier do I need guide uses ratings to set size bands. The best 20 litre dehumidifiers guide compares models in that class.

Compare 20 litre dehumidifiers →

Desiccant ratings work differently

Desiccant dehumidifiers are often rated at a lower figure, typically 6 to 10 litres a day for home models. Their extraction does not drop off sharply as the room cools, so the rating is closer to what they achieve across a range of temperatures. In a cold garage a 7 litre desiccant model can outperform a 20 litre compressor model that is struggling and defrosting.

When comparing types, think about the room temperature first. For a heated home, compare compressor ratings at the same condition. For an unheated space, compare desiccant models with each other. See what is a desiccant dehumidifier and the best desiccant dehumidifiers.

A checklist for comparing fairly

Use the same yardstick for each machine before you look at price or features.

1. 1

   Find the test condition for each rating on the box or maker's spec sheet.

2. 2

   Compare only figures measured at the same condition.

3. 3

   Check the rated power in watts and work out watts per litre at that condition.

4. 4

   Consider your room temperature: compressor for heated rooms, desiccant for cold ones.

5. 5

   Then compare tank size, noise, drainage and modes.

Frequently asked questions

What does dehumidifier litres per day mean? It is the most water the machine removes in 24 hours under a fixed test condition, often 30 °C and 80 % RH. Real extraction at home is lower.

Why does my 20 litre dehumidifier only collect a litre or two? Your room is cooler and drier than the test condition, and the machine stops once it reaches your target. That is normal.

Is a higher extraction rate always better? A higher rating at the same test condition means a more capable machine, but it may draw more watts. Match the size to your home and room temperature.

Can I compare a desiccant and a compressor rating directly? Not fairly. Their output changes differently with temperature. Choose the type for the room first, then compare within that type.