An evaporative cooler, often called a swamp cooler, can cool a Phoenix home with much less energy than central air conditioning. It cools by evaporating water into dry air, so it works best in the dry weeks of summer. When the monsoon brings moist air across the Southwest, a swamp cooler cools much less, while central air conditioning also removes humidity from the air. This guide covers how each system works, how humidity changes over a Phoenix summer, how much water a cooler uses, and when each choice fits.
How an evaporative cooler works
The U.S. Department of Energy's Building America guide on evaporative cooling says these coolers "use evaporation and blowing air to provide cooling." As outside air flows through the cooler, it evaporates some of the water. According to the guide, that can lower the temperature of the air entering the home by 15°F to 40°F. How far the temperature drops depends on how dry the outside air is.
The water that evaporates stays in the air. The guide notes that, unlike standard air conditioning, a direct evaporative cooler raises indoor humidity instead of lowering it.
The same guide says evaporative coolers can use about one-fourth as much energy as central air conditioners. It says they also cost less than half as much to install and operate. Coolers are also rated differently. Air conditioners carry a SEER efficiency rating, but evaporative coolers do not. The guide says no national or international testing organization rates them.
Why evaporative coolers work well in dry Phoenix heat
The DOE guide says evaporative coolers work best in dry climates where outdoor relative humidity is low during the cooling season. It uses Phoenix as its example. Phoenix's 1% cooling design condition, taken from ACCA Manual J, is 108°F with a wet-bulb temperature of 70°F.
Wet-bulb temperature is what a thermometer reads when its bulb is kept wet. In dry air, more water evaporates from the bulb, so the wet-bulb reading falls further below the regular temperature. In Phoenix's design condition, the gap is 38°F, and the guide says evaporative coolers "would work very well" in those conditions.
The guide also gives a rule of thumb. Coolers work best where the 1% summer design wet-bulb temperature is below 70°F. They can still cool where it reaches 74°F, but high humidity reduces how well they work. In those places, an indirect evaporative cooler may be the better choice. Phoenix's design wet-bulb of 70°F sits right at the edge of the best range.
How monsoon humidity cuts a swamp cooler's cooling
The National Weather Service in Phoenix explains in its 2024 Monsoon Season Review that a seasonal reversal in the wind pattern "results in a significant increase in atmospheric moisture across the southwestern U.S." The same review measures the season from June 15 to September 30.
Dew point measures moisture: the higher the dew point, the more water vapor the air holds. NOAA's 1991–2020 hourly normals for Phoenix Sky Harbor put the normal 3 p.m. dew point at 32.1°F on June 1 and 55.7°F on August 1. On those two dates, the normal 3 p.m. dew point differs by 23.6°F.
Normal 3 p.m. dew point at Phoenix Sky Harbor
- June 1, 3 p.m.32.1 °F
- August 1, 3 p.m.55.7 °F
When the air holds more moisture, less water evaporates, and the cooler supplies warmer air. The DOE guide shows this with two examples for an aspen-pad cooler that is about 70% effective. On a 100°F day at 10% relative humidity, the air entering the home would be 73°F, a 27°F drop. On a 95°F day at 50% relative humidity, it would be 84°F, a drop of only 11°F.
How much an evaporative cooler cools the air on a dry day and a humid day
- 100°F outside, 10% humidity27 °F drop
- 95°F outside, 50% humidity11 °F drop
A 2007 article from the University of Arizona's CLIMAS program makes the same point for the Southwest. It is an older, secondary source, so we use it only to back up the DOE numbers above. It says that "during the monsoon, when outside air is already moist, the effectiveness of swamp coolers is limited," and that air conditioning can cool to a set thermostat temperature regardless of the humidity.
Humid heat is also dangerous. The National Weather Service in Phoenix says on its monsoon safety page that "many heat-related deaths occur during the Monsoon as our typical summertime heat is combined with increased monsoon humidity."
How much water does an evaporative cooler use?
An evaporative cooler cools by evaporating water, so it uses water whenever it runs. We did not find a Phoenix water-use figure from a primary source. The numbers below come from New Mexico and from a study published in 2007.
The DOE guide cites that 2007 SWEEP study for a 1,500-square-foot home. Without a bleed-off valve, yearly cooler water use could range from 2,980 gallons in Santa Fe, to 7,261 gallons in Albuquerque, to 12,457 gallons in Las Cruces. All three are New Mexico cities, and none of these figures were measured in Phoenix.
A bleed-off valve drains some water out of the cooler to reduce mineral buildup when the local water is hard. The guide says a bleed-off valve can raise water use by 10% to 50%. For its New Mexico examples, it says to add 50% when the cooler has a bleed-off valve.
Coolers also need regular care to keep the water reservoir clean and free of odor. The guide recommends maintenance at least twice a year: once at spring start-up and once at winter shut-down.
When an evaporative cooler or central AC makes sense in Phoenix
An evaporative cooler fits best when your main goals are lower energy use and a lower installation cost during dry heat. In exchange, it raises indoor humidity, uses water, needs care twice a year, and cools much less once monsoon moisture arrives.
Central air conditioning fits best when you want a steady indoor temperature all summer, including the monsoon weeks. The DOE guide says that in humid climates, homeowners should choose standard air conditioning or heat pumps, which also remove humidity from the air. Phoenix is dry for much of the year, but the NWS says the monsoon brings a significant increase in moisture. NOAA's normal 3 p.m. dew point is 23.6°F higher on August 1 than on June 1. Where humidity reduces how well a standard cooler works, the guide also mentions indirect evaporative cooling as an option.
SRP's Cool Cash rebate table lists per-ton rebates for systems rated 15.2 SEER2 or higher. The rebates are for SRP customers replacing or supplementing an existing central AC or heat pump. New construction does not qualify, and installation must be before April 30, 2027. The table lists $75 per ton for single-stage systems and $150 per ton for multi-stage systems. It lists $225 per ton for variable-capacity systems or inverter-driven mini-splits. These amounts are as of October 2026. The table covers air conditioners, heat pumps and mini-splits, not evaporative coolers. Rebate terms change, so check the current rules with SRP before you buy.