An evaporative cooler, or swamp cooler, blows outdoor air through wet pads; water evaporating from the pads cools the air. An air conditioner moves heat out with a refrigerant cycle. The cooler uses a fraction of the electricity, but how well it cools depends almost entirely on how dry the air is.
An evaporative cooler can only bring air close to its wet-bulb temperature. In dry heat, 105°F at 10% humidity, a cooler about 80% effective delivers air near 74°F; in humid heat, 90°F at 65% humidity, only about 82°F. So evaporative coolers suit dry climates, and air conditioners work everywhere.
Side-by-side comparison
The two machines solve cooling in different ways, so the trade-offs line up cleanly. The figures below are the running examples used throughout this page, at 8 hours a day and 18 cents per kWh.
| Factor | Evaporative (swamp) cooler | Air conditioner |
|---|---|---|
| How it cools | Evaporates water into the airstream, toward the wet-bulb temperature | Moves heat outdoors with a refrigerant cycle, to the set point |
| Climate fit | Hot, dry air only | Any climate |
| Running cost (example) | About $17 a month, a 400-watt unit | About $109 a month, a 3-ton system at 14.3 SEER2 |
| Water use | About 9 gallons an hour at 3,000 CFM and a 25°F drop | None |
| Effect on humidity | Adds moisture; needs open windows or vents | Removes moisture; runs in a closed house |
| Maintenance | Pads, water quality, seasonal winterizing | Filters, coils, refrigerant service |
How cool each gets the air
Evaporation can cool air down toward its wet-bulb temperature, which falls as the air gets drier. Leaving air ≈ outdoor temperature − effectiveness × (outdoor temperature − wet-bulb temperature). With an assumed 80% effectiveness and wet-bulb temperatures from Stull's formula:
| Conditions | Wet-bulb | Air leaving the cooler |
|---|---|---|
| Dry heat: 105°F, 10% humidity | 65.9°F | about 73.7°F |
| High, dry summer: 90°F, 15% humidity | 60.5°F | about 66.4°F |
| Humid heat: 90°F, 65% humidity | 80.3°F | about 82.3°F |
An air conditioner cools to its set point in any of these, and removes humidity as it does.
Running cost
A cooler's main load is its fan and water pump. An example cooler drawing 400 watts for 8 hours a day uses 96 kWh a month, about $17 at 18 cents per kWh.
A 3-ton central air conditioner at 14.3 SEER2 averages about 2.5 kW while it runs, about 604 kWh and $109 over the same hours. Use your unit's wattage and your rate in our kWh cost calculator.
Maintenance and lifespan
A swamp cooler is a simple machine: a fan, a small water pump, replaceable pads, and a water line. As water evaporates off the pads it leaves its dissolved minerals behind, so the pads stiffen and clog with scale and have to be replaced on a schedule. A continuous bleed-off, a small stream of reservoir water sent to the drain, dilutes that mineral buildup, and hard water scales faster than soft.
At the end of the cooling season the cooler is winterized: drain the reservoir and supply line, clean or pull the pads, and cover the unit so no trapped water can freeze and crack it.
An air conditioner trades that water-side upkeep for a sealed refrigerant circuit. The DOE says clean filters can lower an air conditioner's energy use by 5 to 15%, so filters get changed or cleaned on schedule, the outdoor coil is kept clear, and refrigerant work is left to a technician. The DOE puts a central air conditioner's life at 15 to 20 years and a room unit's at 10 to 15.
Water use
Every BTU of cooling evaporates water. Cooling 3,000 cubic feet of air a minute by 25°F takes about 81,000 BTU per hour, which evaporates about 77 pounds of water an hour, around 9 gallons. Over 8 hours that's about 74 gallons a day, plus any water the cooler bleeds off to limit mineral buildup.
Humidity and fresh air
A cooler adds moisture and needs open windows or vents so humid air can leave; in a closed house it soon stops cooling. In dry climates the added humidity can be welcome. An air conditioner recirculates indoor air and dries it, which is what you want where summers are humid; see the ideal indoor humidity level.
A dew-point rule of thumb
Because a cooler can only approach the wet-bulb temperature, its usefulness tracks how dry the outdoor air is, and dew point is the quickest read on that. The National Weather Service calls a dew point at or below 55°F dry and comfortable, 55 to 65°F increasingly sticky, and 65°F or above oppressive.
Those same thresholds work as a cooler rule of thumb. Where the afternoon dew point sits near or below 55°F, evaporation has a lot of room to work: in the dry-heat examples above the cooler drops the air about 31°F (105°F down to 73.7°F) and about 24°F (90°F down to 66.4°F). Once the dew point climbs past 65°F the air is already moisture-laden, the drop collapses to under 10°F (90°F to 82.3°F in the 65% example), and an air conditioner is the better tool.
Coolers are traditional in the arid interior West and the desert Southwest, where summer afternoons run hot but dew points stay low. Through humid Southeast and Midwest summers, where dew points sit in the oppressive range for weeks, a cooler cannot keep up and an air conditioner is standard.
Which to choose
- Evaporative cooler: hot, dry climates where afternoon humidity stays low; low running cost matters; you can keep windows cracked.
- Air conditioner: humid climates, homes kept closed, or anyone who needs precise temperature and humidity control.
- Both: some dry-climate homes use a cooler most of the season and an air conditioner during humid monsoon weeks.
Frequently asked questions
Do evaporative coolers work in humid climates?
Poorly. At 90°F and 65% humidity, a cooler can only bring air down to about 82°F, and it adds moisture.
Is an evaporative cooler cheaper to run than an AC?
Yes, by a wide margin: an example 400-watt cooler costs about $17 a month for 8 hours a day, against about $109 for a 3-ton central AC at 14.3 SEER2.
How much water does a swamp cooler use?
Depends on size and conditions; a cooler moving 3,000 CFM and cooling it 25°F evaporates around 9 gallons an hour.
Can an evaporative cooler replace an air conditioner?
In hot, dry climates, often for most of the summer. Where humidity is high, it can't.
What humidity level is too high for a swamp cooler?
There is no hard cutoff, but the drier the air the better a cooler works. A useful guide is the outdoor dew point: at or below 55°F a cooler delivers a large temperature drop, while above 65°F the air is already so moist that the cooler barely cools and leaves the house muggy.
How often do you replace swamp cooler pads?
It depends on water hardness and run time, because the minerals left behind as water evaporates are what stiffen and clog the pads. Replace them once they are hard with scale, falling apart, or smelling musty, and run a bleed-off to slow the buildup.
Are swamp coolers cheaper to install than AC?
Generally yes, because a cooler is a simpler machine: a fan, a pump, pads, and a water line, with no sealed refrigerant circuit to charge. Central air conditioning adds a compressor, an outdoor unit, and usually ductwork, so it costs more to buy and install.
Do swamp coolers cool as well as AC at night?
In a dry climate a cooler can draw in cooler evening air and still cool well. As the night cools, though, relative humidity usually rises even when the dew point holds steady, which shrinks the wet-bulb depression and the drop the cooler can deliver; an air conditioner holds the set point regardless.
Sources & References
- Stull (2011), Journal of Applied Meteorology and Climatology: wet-bulb temperature from relative humidity and air temperature
- U.S. EIA: Electric Power Monthly (residential prices)
- U.S. DOE: Energy Saver 101, Home Cooling (PDF): filters 5-15%, lifespan 15-20 years
- NOAA/NWS La Crosse: dew point vs relative humidity (summer comfort scale)