The rating is not the capacity
Dehumidifier capacity is quoted at a test condition, and the test condition is warmer and more humid than any basement anyone is worried about. The compressor still runs in a cool space, but the coil pulls less water per hour out of air that has less water in it to begin with, so the pints per day figure drops. How far it drops depends on the machine, and the only honest source is a capacity curve from the manufacturer.
Because that is unknowable in general, the derate is a field. On the defaults — 70 pints rated, 60 percent delivered — the machine is actually a 42 pint unit where it stands. Against a 30 pint a day load that is a run time of 30 divided by 42, times 24 hours, which is 17.1 hours a day. A 71 percent duty cycle, not the comfortable third that the rating on the carton implied.
| Line | Default case | Arithmetic |
|---|---|---|
| Rated capacity | 70.0 pints/day | from the carton |
| Derated to 60% | 42.0 pints/day | 70 × 0.60 |
| Load | 30.0 pints/day | your sources added up |
| Run time | 17.1 hours/day | 30 ÷ 42 × 24 |
| Energy running | 10.29 kWh/day | 600 W × 17.14 h |
| Energy idle | 0.034 kWh/day | 5 W × 6.86 h |
| Cost at $0.17/kWh | $1.75/day | 10.32 kWh × $0.17 |
| Over 180 days | $315.00 | $1.75 × 180 |
| Condensate | 3.75 gal/day | 30 pints ÷ 8 |
Three hundred and fifteen dollars over one damp season, to remove water that a sheet of plastic over the dirt would have stopped for free. That comparison is the reason the ground moisture page is worth running first: on its default numbers, covering the soil takes 57.5 pints a day down to 8.3, which is most of the load on most dirt crawl spaces.
Where the load comes from
The load field wants a number, and there are three ways to get one. Add the calculated sources: ground evaporation and ventilation air are both quantifiable, and on the defaults of those two pages they come to 57.5 and 36.5 pints a day respectively. Or measure it directly, which is better: run the machine to a bucket for two days, empty at the same time each day, and measure what came out. Or work backwards from a machine you already own — if it runs 20 hours a day and you believe its derated capacity, the load is capacity times 20 over 24.
The measurement beats both calculations, because it includes the sources nobody thought of. A condensate line from an air handler that ends under the house, a supply duct sweating in July, a dryer vent that has come apart in the crawl, and a slow drip at a supply fitting all show up in the bucket and in none of the arithmetic.
Reading the shortfall line
When the load is larger than the derated capacity, the page stops reporting a run time and reports how many pints a day the machine cannot reach. That is not a failure of the machine and it is not a recommendation to buy a bigger one. It means the humidity in the space will settle at whatever level makes the sources and the removal balance, and the useful move is usually to attack the source rather than the symptom.
If you want a capacity figure from a description of the space rather than from a load you calculated, the dehumidifier sizing calculator does that, and it is the natural page to reach for when you have no numbers at all. For the reverse problem in winter — a space too dry rather than too damp — the humidifier sizing calculator uses the same grains arithmetic in the other direction. And if the basement is wet rather than humid, start at the water entry organizer instead of at any of them.
Questions people ask
How much does it cost to run a dehumidifier in a basement?
On the default case here — a 70 pint machine drawing 600 watts, delivering 60 percent of its rating, against a 30 pint a day load — it runs 17.1 hours a day, uses 10.32 kWh a day, and at 17 cents a kilowatt hour costs $1.75 a day or $315.00 over a 180 day season. Change any of those four inputs and the answer moves a long way; the wattage and the electricity price are both things you can measure or read off a bill rather than guess.
Why does my dehumidifier run constantly even though it is rated well above the load?
Because the rating is measured at a warm humid test condition and your basement is neither. Capacity falls with air temperature and with humidity, and in a cool space the fall is steep — a 70 pint machine delivering 60 percent of its rating is a 42 pint machine where it stands. If the load is above that derated figure, it runs the whole time and still does not catch up, which is what the shortfall line on this page reports.
How do I measure the actual moisture load instead of estimating it?
Run the machine to its bucket rather than a hose, and empty it at the same time on two consecutive days. What comes out on the second day is the load, in whatever units you measure it — a pint is two cups, and a gallon is eight pints. This beats every calculation because it includes the sources nobody listed: a sweating duct, a condensate line ending under the house, a slow drip at a fitting.
What derate percentage should I use?
Use the manufacturer capacity curve if there is one, because that is the only source that knows your machine. Failing that, the field is deliberately left to your judgement rather than being a constant, because the honest answer varies by machine and by how cool the space is. Run it at two or three values and see how much the answer moves; if the conclusion changes between 50 and 70 percent, the derate is the thing to go and find out rather than the thing to assume.
Can I run the condensate hose outside instead of to a drain?
Sending it outside next to the foundation puts the water back where it started, which for a damp basement is the opposite of the goal. Condensate is also not clean — it has passed over a coil that collects whatever the air in that space carries — so it belongs in a drain. Where a drain is not available below the machine, a condensate pump lifts it, and the discharge should end well away from the wall for the same reason downspout extensions do.