How Much Does It Cost to Run an Air Cooler?

An evaporative air cooler, called a desert cooler or room cooler in South Asia and a swamp cooler in the US, runs only a fan and a small water pump. There's no compressor, which is why it draws so little power.

Symphony rates its 105-litre Sumo 115 XL desert cooler at 185 W and its 23-litre Diet 3D 30i tower cooler at 145 W. We use 200 W as a typical figure for a large room or desert cooler.

At 8 hours a day that's 1.6 kWh: about Rs 53 a day in Pakistan, $0.29 in the US and A$0.76 in Australia. Over a month it comes to about 49 kWh, or Rs 1,610 in Pakistan.

The fan pulls hot air through wet pads. As the water evaporates it absorbs heat, so the air comes out cooler but more humid.

In dry heat the effect is large. The US Building America Solution Center gives the example of air at 100°F (38°C) and 10% humidity leaving a cooler at about 73°F (23°C).

In humid heat it's small. At 95°F (35°C) and 50% humidity, a cooler only brings the air down to about 84°F (29°C).

That makes coolers a good fit for the dry pre-monsoon heat of Pakistan and northern India, inland Australia and the US Southwest. They work poorly during the monsoon and on humid coasts, such as the Gulf's coastal cities in summer.

A cooler needs a way out for the damp air, so leave a window or door partly open. In a closed room the humidity builds up and the cooling fades.

A ceiling fan uses about 60 W but doesn't lower the air temperature. A cooler uses about 200 W and does, in dry weather.

A typical room air conditioner averages about 600 W while cooling, with the compressor drawing around 1,000 W, and a 1.5-ton split AC can draw 1,500 W or more. In dry heat, a cooler costs about a third as much per hour as a typical room AC.

The Building America Solution Center, run for the US Department of Energy, says evaporative coolers can use about a quarter as much energy as central air conditioning.

An AC also removes humidity and holds a set temperature in any weather; a cooler can't. Coolers use water too, which matters where water is scarce or metered.

In Pakistan and northern India, cooler season runs from about March or April until the monsoon arrives in late June or July. In Australia it runs roughly from November to March.

Ducted evaporative systems that cool a whole house are common in Australia and the US Southwest. They have much bigger fans and draw more power than a room cooler.

Because they draw so little, coolers can run on a home UPS during load-shedding. A 200 W cooler runs for several hours on a typical 150 Ah battery, while a 900 VA home UPS can't run most air conditioners at all.

Cooler ratings are the maximum, at top fan speed with the pump running; lower speeds use less. Your cooler's wattage is on its label or in the manual; use that number in the calculator.

• Keep a window or door slightly open so the humid air can escape.
• Use the cooler in dry weather. In humid monsoon heat, switch to fan-only mode or use a ceiling fan.
• Place the cooler where it can draw in outside or well-ventilated air, such as in or near a window.
• Clean the pads and tank regularly and replace torn or clogged pads. Thick honeycomb pads cool more effectively than thin aspen-fibre (wood wool) pads.
• Run it on a lower fan speed when that keeps you comfortable.
• If a cooler runs many hours a day, a BLDC model draws less power.

• Air conditioner
• Ceiling fan
• Dehumidifier
• Space heater
• Bill analyzer