Cost to run an evaporative cooler

An evaporative cooler is a fan and a small water pump, so its draw is a fan’s draw plus a few watts. Manufacturers in this category publish wattage reliably, which makes the arithmetic exact rather than estimated.

Note what is not in that table: a relationship between size and power. The 764 CFM indoor machine draws more than the 2,697 CFM outdoor one. Evaporative coolers are not sold on efficiency and nobody certifies them, so airflow per watt varies by nearly four to one across the category. Dividing published airflow by published wattage: 12.4 CFM per watt for the MC37M, 12.3 for both the CO60PM and the MC61M, and 3.3 for the small indoor CL25AE.

An evaporative cooler consumes water permanently: every drop it uses leaves as vapor. How much depends on how hard it is working, and manufacturers publish tank size rather than consumption rate, so the honest way to price it is by tank-fills.

A 10.3 gallon tank emptied once a day for thirty days is 309 gallons a month. Municipal water in the United States is billed per thousand gallons or per hundred cubic feet, and rates vary enough by utility that a national average would mislead more than it informs — look up your own rate and multiply. For most households the answer lands in the small single digits of dollars per month, which is why this rarely changes a decision.

Two things do change decisions:

• Metered sewer charges. Some utilities bill sewer volume from water volume. Water that evaporates never reaches the sewer, but you may still be charged for it.
• Hard water. Minerals stay behind when the water leaves, coating the pads and the reservoir. That is a maintenance cost rather than a water cost, and it is the main reason pads need replacing on schedule.

The full picture on consumption rates by machine is in evaporative cooler water usage.

Cooling pads are the one part that wears out on schedule. They collect mineral scale and dust, which reduces saturation efficiency — the machine keeps running, it just cools less — and eventually they go stiff or grow biological films that smell.

Manufacturers sell replacement media by model; Hessaire publishes a media set for the MC37M line, for instance. Budget one set a season in hard water and every couple of seasons in soft, and treat a machine that smells as a pad problem until proven otherwise — the diagnosis is in why a swamp cooler smells, and the replacement intervals are in evaporative cooler pads.

At 37¢ a night against roughly 78¢, an evaporative cooler looks like it costs a third of an air conditioner to run. That number is true and the comparison is not, for two reasons.

1. They do not deliver the same thing. An air conditioner removes heat from the room to a set temperature in any weather. An evaporative cooler drops the air temperature by an amount that depends entirely on how dry the outside air is, and adds moisture doing it.
2. The air conditioner is not running continuously. Certified units cycle; an 8,000 BTU model is certified at around 375 kWh for a whole cooling season, not for continuous operation. That is the number to compare against, and it is in what a window air conditioner costs to run.

Where the comparison is fair — a dry climate, a ventilated space, a machine sized to the area — the evaporative cooler wins on running cost by a wide margin and loses on capability. The category-level version of this argument is evaporative cooler versus air conditioner, and the site-wide ranking is in the cheapest cooling to run.