General information from Your Home, energy.gov.au and HealthyWA, not advice for a particular house. The official places to check are Your Home and HealthyWA.
Official sources tie evaporative cooling to humidity rather than to any one city: Your Home says evaporative coolers “work best in climates with low humidity”, and energy.gov.au says they work well in low-humidity inland regions and lose effectiveness on humid days. Perth sits in climate zone 5 on WA’s coastal strip, and none of these sources rates its humidity, so whether one suits you depends on your home and on the air where you live.
Six questions before you choose one
1. How does it cool?
energy.gov.au explains: “Evaporative coolers work by blowing air over a film of water on a filter or sponge, cooling the sponge and the air through the process of evaporation. Outside air is drawn through the evaporative unit (generally on the roof), cooling it via the evaporation process. This air is forced into the house, expelling the hot indoor air through open windows and doors.” A refrigerated air conditioner works with the house closed up, and Your Home notes it “lowers both air temperature and humidity”.
2. What happens on a humid day?
Your Home explains the physics behind the humidity rule: evaporation “works best with low relative humidity (70% or less), because air with lower humidity has a greater capacity to take up the evaporated water vapour than air with high humidity.” energy.gov.au adds that portable evaporative coolers are less effective at cooling a room, and can start to warm it once they have added humidity to the indoor air, unless air flows out of the house.
3. What does it cost in power and water?
On power, energy.gov.au says “ducted evaporative coolers use less than half of the electricity consumed by an air conditioner of similar capacity”, and that models with inverter-driven fan motors use much less power, especially at lower fan speeds. Control varies too: Your Home says smaller and older units have only a fan speed control and no thermostat, while newer whole-house systems can be fitted with electronic thermostats and timers. Before either, Your Home’s first suggestion for keeping cool is a ceiling fan, which it says uses less energy than evaporative coolers or air-conditioners.
On water, the figures are large. Your Home says “Portable units must be topped up with water at a rate of about 4 L/hr. For central systems, water use can be 25 L or more per hour on hot, dry days and this may have implications during water restrictions.” It suggests checking with your council about any restrictions on using water for evaporative cooling.
Your Home also gives the wider comparison: “Generally, high-efficiency reverse-cycle air-conditioning has proved more effective and efficient than evaporative coolers.”
4. What about bushfire smoke?
Because an evaporative cooler runs on outside air, smoke is its weak point. Your Home says “Evaporative coolers bring large volumes of outside air into the house, so when outside air quality is low it may be necessary to limit their use (for example, during bushfires).” Its passive cooling chapter makes the same point: with doors and windows open, they can add to poor indoor air quality during periods of bushfire smoke. By contrast, it describes refrigerated air-conditioning as useful when bushfire smoke gets in the way of ventilating the house.
That is advice about the cooler, not about your health. HealthyWA, the WA Department of Health’s public site, lists air quality among its safety topics; for health advice about smoke, follow what HealthyWA says.
5. What happens in winter?
Perth’s climate zone needs heating as well as cooling, and a roof-mounted cooler can work against the heating. Your Home says built-in evaporative coolers can increase heating bills, because a lot of warm air can escape through the unit’s ducting in the cooler months. “Many modern units have automatic seals when not in use. Otherwise, close off ducts and cover the roof unit in winter to reduce heat losses.” energy.gov.au suggests vent covers to block the cooler’s vents while the heating is on.
6. Is there a health side to the water?
HealthyWA lists evaporative air conditioners among the warm-water settings where one of the Legionella bacteria is found, alongside cooling towers in large buildings, showers and hot water systems, and spa pools. It explains that Legionnaires’ disease is caught by breathing in water or airborne droplets carrying the bacteria, not from other people. HealthyWA links a separate page on minimising the risk of Legionella infection at home from its Legionnaires’ disease page. Your Home’s general rule applies here too: “Service all heaters and coolers according to the manufacturer’s instructions.” In hard-water inland areas, it adds, evaporative coolers are also prone to clogging with mineral build-up.
If you are weighing an evaporative cooler against a refrigerated system, a licensed installer can look at your home, your roof space and your local conditions with you.