A dehumidifier treats the air, not the cause. If water is coming through a wall, from a leak, or from drying laundry indoors, the machine will run forever and your power bill will show it.
A dehumidifier is the only device on this site that removes water from your air at a rate you can measure in gallons. Everything else — vents, fans, absorbers, habits — works in pints or fractions of pints. That makes it the obvious purchase, and it is why so many people buy the wrong one.
Three questions decide it: how cold the space gets, how big it is, and whether the moisture has a source you could remove instead.
- Typical home unit
- 20–50 pt/day
- Works down to
- 65 °F
- Running cost
- $15–40/mo
- Target setting
- 45–50%
Measured at 65 °F and 60% RH under the DOE 2019 method
Compressor. Desiccant keeps going to about 33 °F
30-pint unit, roughly 300–500 W, 8–12 hours a day
Lower settings cost more and buy you nothing
How they actually work
A compressor (refrigerant) dehumidifier is a small air conditioner folded in on itself. It chills a coil below the dew point of your room air, water condenses onto the coil and drips into the tank, and the now-dry air is reheated slightly and blown back out. This is why a compressor unit warms the room a little, and why it stalls in the cold: below about 65 °F the coil starts icing instead of draining.
A desiccant dehumidifier has no compressor. Air passes over a rotating wheel coated in a moisture-absorbing material, a heater drives the collected water off the wheel, and it condenses into the tank. Because nothing has to be chilled below freezing, it keeps working down to about 33 °F. It also puts more heat into the room, which in an unheated garage is a feature.
A thermoelectric (Peltier) mini dehumidifier uses a solid-state chip to make one side cold. It is silent and cheap and it removes 8–20 ounces a day — roughly a pint. That is a closet device. Sold as a room dehumidifier, it is a waste of money.
| Type | Output | Temperature range | Best for |
|---|---|---|---|
| Compressor | 20–50 pt | Above 65 °F | Living space, finished basement |
| Desiccant | 7–20 pt | 33–80 °F | Garage, crawl space, unheated room |
| Whole-house, ducted | 70–100 pt | Above 60 °F | Whole home in a humid climate |
| AC in "Dry" mode | Varies | Above 68 °F | Free if you already run AC |
| Thermoelectric mini | 0.5–1 pt | Above 70 °F | Closet or safe only |
When it does not work
The limits matter more than the benefits — this is where most money gets wasted.
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It will not stop water coming through a wall
Rising damp, penetrating damp and plumbing leaks all deliver liquid water faster than any dehumidifier removes vapor. If a damp patch has a defined edge, sits low on the wall, or gets worse after rain, stop and read structural damp first.
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It cannot beat a moisture source you keep feeding
Drying one load of laundry indoors releases 2–4 pints. If you run a 30-pint machine and dry two loads a day inside, most of its capacity is spent undoing that one habit.
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A compressor unit in a cold garage is dead weight
Below 65 °F it ices up and cycles into defrost more than it dehumidifies. In an unheated space you need a desiccant model, whatever the price difference suggests.
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Drying the air does not warm the wall
Mold grows where the surface is cold, not where the room average is damp. If one corner keeps growing mold while the room reads 50%, the fix is insulation and heat, not more dehumidification.
Doing it properly
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Rule out a water source first
Check under sinks, around the water heater, at the base of exterior walls, and outside at gutters and grading. A dehumidifier bought to mask a leak is money spent twice.
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Measure the space and the temperature
Square footage sets the capacity; the coldest temperature the space reaches sets the type. Write both down before you shop.
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Size it with room to spare
Use the sizing calculator. An oversized unit reaching its target and idling costs less to run than an undersized one running flat out all day.
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Set it to 45–50% and leave it
Every 5% lower costs meaningfully more electricity. Below 40% you are drying wood and skin for no health benefit.
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Plumb the drain if you can
Most units take a garden hose on a gravity drain. Emptying a tank twice a day is the single most common reason a dehumidifier ends up in the garage unused.
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Give it air on all sides
Keep it clear of walls and furniture and put it near the damp, not in a doorway. Close the windows in that room while it runs.
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Re-measure after two weeks
If the reading has not moved, the machine is losing to a source you have not found yet. Go back to step one.
What it costs
Prices are US retail ranges in 2026 and running costs assume roughly 16 cents per kWh. Treat them as the shape of the decision rather than a quote.
| Unit | To buy | To run | What it suits |
|---|---|---|---|
| 20-pint compressor | $140–$200 | ~$11/mo | 250 W, 8 h a day — a single damp bedroom |
| 30-pint compressor | $180–$280 | ~$17/mo | 350 W, 10 h a day — the size most basements want |
| 50-pint compressor | $240–$350 | ~$29/mo | 500 W, 12 h a day — large or very wet spaces |
| Desiccant, 15–20 pint | $200–$320 | ~$35/mo | 600 W and constant — the cold-space option, not the efficient one |
| Whole-house ducted | $1,200–$2,500 fitted | $25–$60/mo | Only earns it when the entire house runs humid for months |
How to tell it is working
A dehumidifier that is working looks wrong for the first week. The tank fills fast, then fills slower, and people read the slowdown as a failing machine when it is the opposite. The first days pull water out of the air and then out of everything porous in the room — plaster, timber, carpet backing, stored paper and cardboard. Once those have given up their moisture the unit is only handling what the room produces each day, so the tank fills far less often. A slowing tank is the sign of success.
The measurement that settles it is the room reading at a fixed time each morning, taken before anybody has opened a door or run a shower. Expect the number to fall three to eight points in the first 48 hours and then to drift down slowly for two to six weeks as the building fabric dries. If the reading has not moved at all after two full days with the door shut, the machine is not the problem — you have a water source it is fighting, or the space is too cold for the coil.
Two failure signs are worth knowing. Ice on the coil means the room is below about 65 °F and a compressor unit will keep drawing power without collecting much; that is the case for a desiccant model rather than a bigger compressor. A unit that runs permanently without ever satisfying its humidistat, in a warm room, means liquid water is arriving from somewhere — stop and find it before the electricity bill does the arithmetic for you.
What to check before you commit
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Check the rating year, not just the pints
A 2019-or-later pint figure is measured at 65 °F. Older stock and older reviews quote the 80 °F figure, which is about a third higher for identical hardware. If a listing brags about 70 pints at a price that seems too good, it is almost certainly quoting the old test.
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Look for a continuous drain port
A gravity hose to a floor drain or a condensate pump turns the machine from something you empty into something you forget about. A full tank stops the unit, usually overnight, which is exactly when you wanted it running.
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Check the humidistat has real settings
Some cheap units offer only "high, medium, low" rather than a percentage. Without a target you cannot set 50% and walk away, and the unit either overshoots into needlessly dry air or never gets there.
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Match the low-temperature behavior to the room
Auto-defrost extends a compressor unit down to about 41 °F but with a large efficiency penalty. For a genuinely cold garage or crawl space, a desiccant unit is the honest answer even though it costs more per pint.
Common mistakes
Buying on tank size
Tank capacity tells you how often you empty it, not how much water the unit removes. Pints per day is the only number that matters.
Setting it to 30%
It will run almost continuously, cost far more, and dry out wood floors and furniture. 45–50% is the target.
Running it with a window open
You are then dehumidifying the outdoors. Close the room while it works.
Using it instead of a bathroom fan
A shower dumps moisture in fifteen minutes. An exhaust fan removes it at the source in real time and costs a fraction to run.
Common questions
What size dehumidifier do I need for a basement?
For a typical 1,000 sq ft basement that smells damp, a 30–35 pint unit under the current DOE rating is the usual answer, moving to 50 pints if there is standing dampness or the space is above 1,500 sq ft. Use the sizing calculator for your numbers.
How much does a dehumidifier cost to run?
A 30-pint unit draws roughly 300–500 watts. At 16 cents per kWh and 10 hours a day, that is about $15–25 a month. Larger units and lower humidity targets push it toward $40.
Should I run a dehumidifier all the time?
Run it on a humidistat rather than continuously. Set 45–50% and let it cycle. Continuous operation at a low target is the most expensive way to get an outcome you did not need.
Can a dehumidifier make mold worse?
No, but it can hide the cause. Drying the room air lowers the surface humidity on cold walls a little, so growth slows without stopping. If mold keeps returning in the same spot, the wall is too cold — that is an insulation problem.
Where should I position a dehumidifier in a room?
Somewhere the air can circulate around it — near the middle if that is practical, and at least six inches clear of every wall and piece of furniture. A unit pushed into a corner dehumidifies the corner, because it recycles its own dry output rather than reaching the rest of the room. Doors and windows shut, or you are trying to dry the whole house with a room-sized machine.
Can a dehumidifier make a room too dry?
A unit with a working humidistat set to 50% will not, because it stops when it gets there. A unit run on continuous mode in a small sealed room can pull the air into the twenties, which dries out joinery and airways for no benefit at all. If you are running continuous mode because the humidistat is unreliable, that is a reason to replace the machine rather than to accept the setting.
Sources
- US EPA — Mold and moisture guidance — the 30–50% range and the 10 sq ft remediation threshold
- ASHRAE Standard 55 — Thermal environmental conditions for human occupancy — indoor comfort and humidity limits
- US DOE 10 CFR 430 — Dehumidifier test procedure — the 2019 move to 65 °F rating conditions