Is it more economical to leave the air-con on?

MythDefied
21

Depending. It seems in my experience that if you’re home more than you’re out, AND you have an inverter system, AND the temperatures are what we in Australia would consider somewhat extreme, leaving the units on seems to give better results than playing catch-up. I’ve proven to my satisfaction (using AGL’s web based smart bill thing) that even when I left an inverter system on 24/7 in a not so well insulated house, it was fractionally cheaper than turning the unit off and on. (We’re both retired and so we spend significant amounts of time at home, but this would also apply to home offices and businesses I guess.)

Interestingly, by far the biggest chunk of energy usage we had was the electric hot water system, which is set to use off-peak power. Fully a quarter and sometimes up to a third of the daily energy bill occurred during the period when the HWS was on. And on non-timed system where the tank was outdoors with no other mitigating insulation, I was able to observe that to come on for up to eight times a day in winter for fairly long periods even when I was using no hot water at all, so old HWSs would seem to be a worse issue. I imagine that using a well-insulated heat pumped HWS would reduce bills more than mucking around with whether to have the AC on all day or only when home…

It seems to me that just having access to better information about one’s energy usage is an eye-opener in that you can then see these things for yourself and make adjustments.

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22

I wouldn’t be so sure. Maybe it depends, maybe not.

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23

Thanks for rising to the challenge here everyone. The problem with myths is that many of them are either vague, or situation dependent. To solve the question, we often have to put forward cases to illustrate what is the best thing to do in a particular scenario.

For someone like @teddlesruss, you can see why it makes sense to leave the A/C on, and they also give the excellent advice about using access to energy info if you have it to your advantage. For those in moderate climates or leaving the house for longer periods, it would seem that switching off is the way to go. CHOICE also provides some air conditioning tips that might be helpful.

Please keep adding in your experiences - no doubt it will help inform others searching for the same answer.

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24

Don’t forget the fur babies! We generally leave the A/C on 26° C in summer when we go out for the day.

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A refrigerator type AC is a heat pump - it doesn’t “fight” heat it just uses electricity to run a pump that utilises venturi effect coupled with ‘hot side’ and ‘cold side’ of a closed system. Fans and air circulation patterns can make AC units more effective (even non-inverter ones).

Studies have been done which measure the electrical energy consumed by keeping the AC running when there are no people in building versus the electrical energy required to do the initial pull down of temperature when the AC is turned on after it was off allowing the interior of the building to heat up.
The study I know most about was indeed done before inverter technology was applied to heat pumps - but that would not affect how the AC runs to do the initial pull down of temperature when it is switched on for a building that has been allowed to heat up (inverter technology comes to the fore in a small temperature range around the plus/minus of the thermostat setting).
==> the study found that running the AC 24 hours per day did use less electricity (measured over weeks and months) because that initial effort to pull down the temperature after the inside of a building has been allowed to heat up is very significant.

All of the above is simply from the point of view of “amount of electricity used” but others have made valid comments about human comfort when you re-enter a building, as well as the timing of energy consumption (cost per kW at different times of the day, and peak load times on grids when people get home and turn on lights & devices & start cooking tea etc).

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26

I’m with @PhilT on this one. I’m a shift worker and a very hot sleeper. My house is double brick and after a string of hot days without cool nights to balance out, I can feel the heat emanating from my bed and walls. Even the concrete slab is an easily forgotten significant mass of heat storage.

If left off long enough, my aircons have to work exceptionally hard for an extended period to remove the built up heat from all the solid objects. More economical or not in my situation, apart from being hard to quantify, is moot. A restful sleep for me is critical in managing night shift fatigue and possibly a resulting accident.

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27

Our previous residence was a 3 level rendered masonry block house located on the Northern side of a hill so we did not receive the prevailing breeze.

In summer time, the water from the cold water tap at the kitchen sink which was on the Eastern side of the house would be unpalatably warm at 9:00 AM until allowed to run for some time.

The Western side of the house where our master bedroom and ensuite were located received the full afternoon sun.

The house had insulation installed under the roofing iron when we had it built.

The kitchen and dining room on the Eastern side both had Fujitsu ASTA24CMCA inverter split systems installed, the master bedroom had a Fujitsu ASTA12CMCA inverter split system installed, and the other 3 bedrooms and the office had Fujitsu box aircons installed.

The costs of running the 3 split systems and 2 fridges was very high, and the highest consumption was on a summer day when our elder daughter and her children were staying with us. It hit 93kwh.

Even when we were away on holidays at Xmas for around 4 weeks with just 2 fridges and a few CCFL lights left on, the daily consumption was still around 22kwh.

When we inspected our current home at 5:00 PM in mid-January, 2015, we were amazed as to how cool it was after having been locked up all day. The original owner had moved out some weeks previously.

It is a single level rendered masonry block house on the South side of the hill and it faces the prevailing weather.

It has both insulation installed under the roofing iron and insulation on the ceiling, and 2 rotary ventilators mounted on the roof.

We have a high hedge close to the Western side and trees near the Northern and Eastern sides of the house.

We have exactly the same model Fujitsu aircons installed in the kitchen, loungeroom and master bedroom, as well as an old Fujitsu split system in one bedroom and a Fujitsu box aircon installed in the other bedroom.

When we were away for around 4 weeks last Xmas with just the fridge and a few LED lights turned on, the average daily consumption was just 4kwh.

Our average daily consumption is around 22kwh in Summer and 10kwh in Winter, plus under 4kwh for the swimming pool, and our power bills are around half of what they were at our previous residence.

We have a Cent-a-Meter which was left when we bought the house which I managed to get operational so we can see the current power usage at any time with the display in the kitchen.

I also read the electricity meter and record the consumption at around 5:00 PM each day.

I don’t believe that we would be better off by leaving the aircons running at our presence residence and we most certainly could not have afforded to do so at our previous residence.

We have done everything possible to minimise our electricity consumption with the exception of installing solar power to date.

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28

Unless you had something else running in the background like a pool pump or electric hot water with a leak, this smells of a grossly over-reading power meter. Two large fridges and a few CFL’s should consume well under 5kWh per day.

Our farm property with 3 fridges, 3 freezers, 2 evap aircons, 2 reverse cycle aircons (used sparingly), TV, lights, dishwasher, workshop equipment, bore pump and at times a cool-room averages 22kWh per day during the peak season. There is no solar power subsidising this.

29

I have a Mitsubishi 743L multi-drawer fridge/freezer, still made in Japan :open_mouth: and utilises an inverter compressor. Stable temperatures, very efficient (it would well suit off-grid systems), idles down to 75W and has a start up peak of 125W making it effortless to start from my small inverter generator. In contrast my previous Samsung fridge demanded 1250W to start.

Maybe Choice could add the MR-WX743C to their testing if it isn’t in the pipeline already @BrendanMays as appliances suitable for off-grid use are becoming more sought after.

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30

An interesting article regarding air-conditioning and appliances in the tropics.

https://www.abc.net.au/news/2018-11-10/using-your-aircon-better-to-cool-a-home-in-the-tropics/10477238

31

As a physicist who has studied air-conditioning since 1979, and who last year bought my fifth reverse cycle air-conditioner, I can confirm that the most economical way to operate an air-conditioner is to set it to come on in time to bring the room to a comfortable temperature when you intend to use the room. If it comes on any earlier, it will be using electricity for no purpose.

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32

True for some, but is it always the best strategy?

Depending on house construction, climate (humidity), and rooftop solar. How does one take into account the benefits of providing a more stable internal environment? Given the prolonged wet seasons here on the east coast and mould invasions, RC air conditioning may be the only cost effective preventative measures some of us can rely upon.

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I apologise that my advice was based on comfort, and did not address mould prevention.
If you have a mould problem that needs the air conditioner to operate even when nobody is home, you may be able to minimise your electricity use by raising the set-point by a couple of degrees and operating the air-conditioner in “dry” mode.

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And thanks too you fr the link… I was about to do a search, had never heard it either :slight_smile:

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Comfort in our home includes the humidity (it can be too high or too low), room feel and aroma.
It’s not just mould management. With higher humidity paper in the printer goes all limp and sticky. Any cloth covered seating including lounges takes on a damp towel feel, nothing inside ever seems dry to the touch, and the rooms take on that wet dog musty aroma in varying degrees. It takes more time to dry the room to a comfortable level, than it does to adjust the temperature.

Does the drying mode on a split system AC save any power and is it more effective than a dedicated dehumidifier? Our experience of several homes in different climate zones is it depends.

The best saving tip from our experience. If your home is sub-tropical to tropical or in the hot dry inland - install rooftop solar PV. In which instance it’s less important to worry about small savings in power use. Our strategy on hot days had been to cool the house a few extra degrees before the sun goes down.

In North Qld without access to solar PV and Ergon the retailer/distributor. We connected our RC AC to the lower cost tariff 33 regulated peak demand supply. Combined with cool tiled slab on ground floors and ceiling fans the evening power interruption passed with little concern.

It’s worth considering the greater number of existing homes are not well sealed for air tightness, and if older lack effective moisture barriers. The ability and costs to remedy are different in every instance. There is a point where the cost of further home improvement offers no justifiable return, vs investment in better heating and cooling, IMO.