Evaporative cooling running costs: electricity, water and the summer bill

An evaporative cooler has two meters, and only one of them is the story people tell at the hardware shop. Electricity is usually modest: a fan to move air and a small pump to lift water. Water is the working fluid. On a dry Perth afternoon the machine can evaporate around a hundred litres an hour. That is not a fault. That is the cooling.

The summer bill is therefore a joint account. Households that compare a rooftop cooler with a reverse-cycle system on power alone will make the cooler look like a bargain. Households that ignore water will be surprised in March. The honest method is slower and uses published ranges, the hours you actually run, and the tariffs on your own bills.

What you are paying for on the switchboard

A refrigerated or reverse-cycle air conditioner spends most of its electricity on a compressor. An evaporative cooler does not have one. The large draw is the fan that pulls outdoor air through wet pads and pushes it down the ducts. The pump is a minor extra. On humid days you can turn the water off and run fan-only; the power bill barely notices, and the rooms stop gaining moisture they cannot use.

Nameplate watts vary with the size of the unit and the age of the motor. Read the label on your machine rather than a national average that does not exist in a useful form here. Older ducted units often have no useful thermostat. They run until someone switches them off, which means the hours in your estimate should be the hours the switch is on, not the hours you felt warm.

A whole-house ducted cooler still tends to undercut a ducted reverse-cycle system on electricity, especially when the refrigerated unit is cycling hard through a 40-degree afternoon. That comparison is the one evaporative versus reverse cycle exists to unpack. It does not settle the water question.

Portable coolers are electrically cheap and hydraulically small. They serve rooms up to about 25 square metres if the room can exhaust air. Wall and window machines cover a larger room, roughly 30 to 50 square metres. Neither will match a ducted rooftop unit on either meter, because neither is cooling the house.

The published water ranges

Water use is not a single number. It is evaporation plus bleed-off, and evaporation tracks heat, dryness and how hard the fan is running.

NSW Planning has put typical evaporative use in a band of about 10 to 30 litres an hour, depending on size and humidity. That is a planning figure for milder or more humid conditions, not a Perth January.

Water Corporation in Perth has said that around 100 litres an hour can evaporate in hot weather, and that a household running six hours a day for 50 days can approach 30,000 litres a year from the cooler alone. Those are the figures to carry if you live in a dry western summer and you actually use the machine.

CHOICE, citing a 2009 study, reported 41 to 70 litres an hour in Hobart and 67 to 116 litres an hour in Adelaide. Hobart is not evaporative country in the Perth sense; the Adelaide band is the one that matters for a large stretch of South Australia. The spread inside each city is the point. A mild evening and a 44-degree afternoon are not the same litre.

Bleed-off is extra — the water quality and bleed essay is the workshop version. Many older systems dump 11 to 20 litres an hour to keep the tray from turning into a mineral pond. Newer controls can bring that down to around 3 litres an hour. Water Corporation’s field check is homely and useful: a bleed that half-fills a bucket in 30 minutes is about 11 litres an hour. If your hose fills a bucket in ten minutes, you are not saving the pads. You are watering the garden through the roof.

How to estimate a household summer

Start with hours. Not “all summer”. The hours the unit actually runs. A household that switches on after school and off at bedtime is not a household that leaves the cooler on from breakfast. Older machines without a thermostat make this a behavioural number. Write it down for a fortnight and stop guessing.

Electricity is arithmetic. Fan watts plus pump watts, divided by 1,000, times hours, times your cents per kilowatt-hour. Use the rate on the bill, including any daily supply that you already pay whether the cooler runs or not. The cooler’s share of the supply charge is not a useful number. The usage is.

Water is a range, not a point. Pick a band that matches your climate. Perth-class heat: treat 100 litres an hour as a hot-afternoon rate, not as a 24-hour average, and use a lower figure for mild days. Adelaide: the 67 to 116 litre band is a published starting point. Inland New South Wales or western Queensland on a dry week will sit toward the high side. A humid coast should not be running the water at all.

Multiply the hourly figure by your hours. Add bleed at the rate you can measure with a bucket. Convert to kilolitres and apply your water usage charge. If your retailer also charges for wastewater on a formula that includes indoor use, find out whether cooler water is treated as indoor use. That varies, and inventing the rule would be worse than telling you to read the tariff.

Do the sum twice: a mild summer and a hard one. The difference is mostly hours and evaporation rate, not a mysterious third meter.

What the comparison with reverse cycle actually costs

Reverse cycle spends more on electricity and, in a sealed house, nothing on water for the cooling function. Evaporative spends less on electricity and something real on water. In a dry climate with open windows, the evaporative bill can still be the smaller one. In a climate that is already humid, or under water restrictions, the water line can erase the power advantage.

Insulation and shading change both bills. A cooler that is fighting an uninsulated dark roof will run more hours for less comfort. That is hours on both meters. The essay on insulation, shading and passive cooling is the cheaper intervention.

Bleed that is too generous is a water cost with no cooling benefit. Bleed that is shut off is a pad cost deferred. Neither is free.

The habits that move the number

Run the water when the outdoor air is dry enough to take it. On a muggy change, switch to fan-only or switch off. You are not being frugal. You are stopping the machine from adding moisture it cannot evaporate.

Do not cool empty rooms through open ceiling vents if you can close them. The fan still draws, but you are paying to push air into space you are not using.

Fix the building before you buy a larger unit. West glass, a tin roof and no pelmets will outspend a new motor.

If you are on a time-of-use tariff, the cooler’s cheap power is still cheap, because it was never the expensive appliance. The expensive decision is leaving it on overnight out of habit when the wet-bulb has already come up.

What not to trust

Brochure “cents per hour” figures assume a water price, an electricity price and a climate that may not be yours. Retail rankings that ignore water are half a ledger. A neighbour’s bill is an anecdote about their hours and their bleed.

Public sources — NSW Planning, Water Corporation, CHOICE’s cited study — give ranges. Ranges are the honest object. Your meters settle the rest.

A summer of evaporative cooling is a known trade: a modest power draw and a visible water draw, in a house that is allowed to breathe. Price both, or the bill will do it for you.