Being Smart about Power Bill on Air Conditioner: Keep Temp at 24 ~ 25c

I'm trying to understand how to avoid swollen electricity bill, particularly on air con.

Folks at my work said to keep it at 24 ~ 25c is the key; his example was keeping your house consistently cooled down to 24 ~ 25c from the morning (even when it's not hot) will reduce the workload of your aircon as it doesn't need to work that hard and when it gets to peak afternoon, the house is already at 24 ~ 25c hence aircon won't work as hard.

is there much accuracy in the above method?

what my colleagues are saying is that, when comparison is done on quarterly basis, purely on aircon:

  • running aircon 24x7 at consistent 24-25 centigrade versus running aircon 5 hours (daily) at 19 centigrade

the latter would incur more swollen bill.

what's your thoughts?


  • +1 vote

    It takes less energy to maintain the temperature (especially one which is much closer to the ambient temperature) than to reduce the temperature significantly.

    Same principle as it takes less fuel to maintain 100km/h than it does to accelerate to 100km/h.

    • +1 vote

      It takes less energy to maintain the temperature (especially one which is much closer to the ambient temperature) than to reduce the temperature significantly.

      in that case, if at 11 pm tonight temp drop to 18 degree; running aircon at 24-25c will be more expensive than running it at 18c ?


        Assuming it's not a reverse cycle setup, the compressor shouldn't turn on if ambient is cooler than the temperature you've set at.


          What type of aircon would be perfect for this?

          I really am thinking about investing in a 24-25c 12/7 aircon for when I am not at work and at home and asleep also my pc will love me.

          I have a really small room to cool like think two small bathrooms put together and that is my current room.

          Was considering a portable aircon but my window is really difficult to work around as it is an old house and I am not sure what would be the cheapest to own and run mainly run over the long term like 10-20 years.

          So to recap which aircon is best for a small room to keep cool at 24-25c 24/7 or at minimum 12/7.


            @AlienC: Probably should consult an expert if you are looking for optimal solution since I had some installers come out to give me advice on my apartment and they took into consideration not just room size but things like direction of sun, window locations, materials in building, room height, layout, balcony and shade, window thickness, etc.

            Otherwise I think most people just get an energy efficient split system.


              @jace88: Can recommend any specific installers in Sydney or company in general or how should I go about it to get the best of the best and still a good deal?


                @AlienC: I went with Alliance Climate Control. Highly recommended and they’ll come out to do a free quote for full supply and fit. They weren’t the cheapest but they had a no surprises approach and no overruns.

                If your needs are relatively simple (ie a single room next to a wall where you’ll install a compressor outside) then you might be able to get sufficient advice from a big box retailer. Honestly this will probably be more cost effective upfront if you’re trying to get the best deal upfront vs the most optimal/efficient solution in the long run.

        • +1 vote

          Even a reverse cycle A/C won't run the compressor if the room is at/cooler than the set temperature as long as you have it set to cooling only.

          If you have it set to auto (or equivalent) it will operate in reverse and heat the room, as it is now trying to bring the room up to the setpoint rather than just trying to keep the room below it (as it would for cooling only)

          Vice versa for heating.

    • +5 votes

      Thats for instantaneous power consumption, though. In terms of Kwh's, maintaining a room all day at 25 would almost definitely use more energy than turning it on for a few hours.


        Realistically it would only be for extreme hot days so this summer has been pretty hot (today is a rare exception) so some nights I can use the ac to get a good night's sleep.

        Also I feel it might help my pc on some brutally hot days.

        But I would only be using it some parts of the year as it gets pretty cool in the blue mountains other times of the year.


          My reply was more directed at jace88's claim, but to answer your question on which AC to get, it would depend first on how long you plan to stay in your current house. Less than a year or two? Definitely portable. Or an older window box, if your handy and live on the first story. Anything past that, and its probably worth getting a high efficiency 2.5kw unit from a reputable brand. Cost you about $1200 if you get it on sale and pay for installation separately.

          It would definitely be good for the PC. I didn't realize why so much electronics would randomly break, but it turns out everything's only really rated for operating conditions of less than 30c. Anything more than that, and you rapidly increase the risk of a fault occurring. It's one of the reason why I want to keep my house set to 25c too. Of course, running the AC that much will lower the humidity, so you have to watch out for static buildup more. They have that problem in data centres.

          • +1 vote

            @outlander: Great comment on static I always forget how humidity and static directly relate to each other.

            AC on means low humidity so more dry air coming through increasing the chance of static which I honestly hate more than many things.

            In fact the static comment is now making me rethink an air conditioner near my pc or in the same room as my pc.

  • +3 votes

    Why do you need to run at at 19c? Why not just 5hr at 24-25c?

    • +1 vote

      Why do you need to run at at 19c?

      some people like their room/house chilled, like real chill. i know some folks that like to run their aircon at 16c.

      • +29 votes

        I know what you mean, bro.

        • +4 votes

          can you let the person who negged me know that chill does exist?

      • +1 vote

        Those people don’t care about power costs.

  • +1 vote

    Cooling - save up to 10% for each degree that you increase the air conditioner’s temperature.

    We always run ours at about the 24-25. In our house, if we try and put it on about 22 or lower on one of those hot days you can hear the aircon working harder and it never has a break. At 25 it sometimes gives itself a break every now and then when set to auto fan speed.


      but then if temp is 47, like NSW today .. running at 25c be more expensive than running at 27c as it works harder by 2 centigrade?

      if temp cool down to 18 like 11 pm tonight .. running at 25c be more expensive than running at 18c.


        we sometimes set ours to 26 or 27 if we feel a bit too cold ;)

        if temp cool down to 18 like 11 pm tonight .. running at 25c be more expensive than running at 18c.

        our system will sit idle if it's cooler than 25. however, if it's 18 outside we'd open up all the doors and windows anyway.

      • +1 vote

        If you have your AC set to cool to 25C and the outside temp is 18C, the compressor of your AC will not be operating, only the fan to circulate the air. So no, it won't be more expensive running with the thermostat set at 18C.

  • +5 votes

    keeping your house consistently cooled down to 24 ~ 25c from the morning (even when it's not hot) will reduce the workload of your aircon as it doesn't need to work that hard

    It will depend. My AC will work 'hard' for about an hour and then the house is cool. In that hour it will use 2-3kw of power.

    If I turn it on at 9am instead of 2pm when I need it, I'll use way more than 3kw of power between 9am and 2pm.

    So in my case, its cheaper to wait.

    People who say turn it on early to get it cool and keep it cool, generally have under sized units that struggle.


      You have an ac that only uses 3kWh of power to cool your entire house?

      What kind of AC is only 3kw draw for whole house cooling?


        The kW rating that is generally advertised is actually it's cooling capacity (how many Watts of heat it can transfer) not it's energy consumption. If you compare the two, A/C units are highly efficient - they will use a fraction of a kW to move a kW of heat.

        So for example a 3.5kW unit will use around 1kW of electricity to actually transfer that heat. So if it's running at 100% (the whole time) it'll use 1kWh every hour while running (at 100%).

        3-4kW of energy usage would be a 10-15kW capacity system.


    Mine is 26-28C.


    Theoretically speaking vs in reality.

    I only use it if i have to and its around 23 for 2-3 hrs at night before i sleep;just enough to fall asleep

  • +1 vote

    Put aircon on 27 when used


    get a clamp meter and work it out.

    I think you should have it on 24-25 for the hottest part of the day, and leave it at that.

  • +1 vote

    We have central heating/cooling system in the living room and other rooms, so being tight-asses we found an ultra-cheap option to let the "fan only" option work from 9PM to 9AM (when outside is the coolest). Drops the temp by a few degrees, costs the same as a standing fan.
    Though we are not 100% sure if it is getting air from outside or it is just circulating the inside air… too lazy to climb on the roof to check.


      Yes - using "fan only" is a great option and is usually sufficient.

      It's much quieter than using a standing fan and I wouldn't be surprised if its energy consumption is cheaper.

      Re temp: a year after I had aircon installed (Daikin), I actually read the instructions and saw the recommended temp to be set for cooling is 26 - 28, which sounds high but seems to work for the horribly hot days

      Don't forget to clean the air filters regularly. Dirty ones waste energy.


      I don't get how this would cool the house at all?

  • +3 votes

    Every degree makes a difference. We set it's to cool to 26 or 27c in summer and heat to 20 in winter. If you are not renting, and you are probably not if you are thinking about installing a split system, think about getting an iotawatt installed in you electricity metre box. It will be able to tell you exactly how much power your AC and other circuits are using and when so you can verify the theory about running all day vs only peak. Your AC installer will be able to install the iotawatt at the same time. We notice that our units all use a lot of power initially and then drop down to a much lower stable amount (half or less) to maintain, but it still might not be cheaper to run it all day. If you also own your house, consider installing solar and a great pump hot water system. We went from paying 200 per month to them paying us about 180 per month and since our system produces more power than we are allowed to export, the excess usually covers all our daytime usage (computers, air con, hot water system) which is therefore all completely free now. The system will pay for itself in about 5 years.

    • +2 votes

      Here's a screenshot of the kind of data you can get from the IoTaWatt.

      Everything below the grey line is "free" (self consumption of solar power in excess of our export limit). The blue spike is the heat pump hot water system and the red spike is air con while our kid was having a nap in the bedroom. The spike at the start of AC usage is low in this example, but you can see that in the early afternoon when this room is hottest it goes for about half an hour at ~776W and then settles down at ~616W. Multiply those by your utility rate to work out if it's more efficient to run all day or only when you get hot.

      12 hours runtime:

      616W / 1000 = 0.616kW * 11 hours * $0.31c (peak rate, assuming no solar) = $2.10
      776W / 1000 = 0.776kW * 1 hours * $0.31 = $0.25
      $2.35 total

      vs 4 hours total runtime with bigger initial spike:

      616W / 1000 = 0.776kW * 3 hours * $0.31 = $0.57
      1232W / 1000 = 0.776kW * 1 hours * $0.31 = $0.38 (assuming worst case where initial spike is DOUBLE ongoing usage, which is not the case in my real world example above, even starting at peak time)
      $0.95 total

      Here's my review of the IoTaWatt too:


        That's a very informative post. Thanks for the level of detail you've gone into.

        I've read your review that your linked - question: does the system monitor batteries also? Separately, done your arms a tech with similar interests to myself… have you got any ability to monitor individual power points and if so how have you managed that?


          I use several TP-Link hs110 smart switches with energy monitoring for specific power points I want to monitor. One for the home office (computers) one for "chargers" (as batteries, USB power bank, universal remote, robot vacuum, laptop, etc), one for home theatre (sub, amp, tv, consoles, streaming boxes, etc), one for the fridge which is not on its own circuit.

          I run home assistant on a cheap odroid c2 system on chip I had lying around. You could use a raspberry pi or similar. It pulls in data from the iotawatt AND TP-Link switches to graph them all together (though I prefer iotawatt graphs). And I use home assistant to automatically turn on the office at 830 am, turn on chargers when solar production exceeds our export limit, and automatically turn off chargers, office and home theatre when solar production stops and consumption is at standby levels.

          I use a harmony hub/elite remote to turn on the home theatre switch and amp and tv etc when I want to watch something.

          Iotawatt also exports data to PVOutput which has excellent graphs and I monitor 6 sub circuits there, but the local iotawatt graph is higher precision so I have basically recreated the PVOutput graphs in iotawatt. But the best thing about PVOutput with solar is that I knows your rates and supply charges and gives you a +/- solar value per day, week, month, etc.

          The solar system is now generating us income, but you don't need the solar system to save money. Just having the iotawatt and a couple of smart switches will give you the Intel to see where you can cut down your usage and how much you can save. Just having my sub alone off instead of in standby for example is saving about $85 a year. And having home assistant gives you the convenience of automation and aggregate reporting.


          Oh, I don't have a battery. But the iotawatt will monitor any AC circuit in your meter box. You can run the CT clamp further with an extension cable and possibly monitor the AC connection to the battery. But more likely your battery will have some kind of data export or API that you can bring into home assistant.

          HA also can run many add-ons like grafana and influxdb to visualise your aggregate data, plus handy things like a VPN and pi-hole for network wide as blocking etc.


            @mrmachine: Awesome, many thanks - I've had a look @ PVoutput many times over the years. Seems like a great project.

            I've just bought 20 sonoff minis and have a couple of the brilliantsmart plugs from the OW price matching Bunnings a while back. I'm intending to de-cloud them and was looking at HA as the coordinator. I've got several board computers kicking around, several KW of lithium cells and 5 kw of second hand panels. Currently running a single clamp monitor on the fusebox, but it's "dumb" and just irritates me. Also has a proprietary app, which also goes against it.

            What sites would you recommend for a near-noob to get up to speed? Are there any YT channels you recommend?


        This sounds interesting. I built my own Raspberry Pi based solar and consumption monitor, but this looks much more capable in that it monitors individual circuits (mine monitors the solar inverter RS232 port and two power meter flashing LEDs).
        Does IoTaWatt require an electrician to install?
        What components are required to get an up and running, monitor-able system?

        • +1 vote

          You can install it yourself if your metre box already has a GPO to power the iotawatt and reference voltage brick. It's just CT clamps which do not require any rewiring behind the metre board, but you will need to open up access behind the metre board to clamp the circuits. Make sure the main breaker is off and be aware that the net mains cable is still live. If in doubt get a sparky to install. And use Velcro to mount the iotawatt in case the power company needs to move or remove it.

          All you need is the Aussie bundle with enough CTs for all the circuits you want to monitor, plus the locally sourced USB and reference voltage bricks that they recommend from radio parts.

          The iotawatt forum is excellent and the developer is extremely helpful with installation and usage. The iotawatt itself is pretty simple. PVOutput is a bit more complicated to get your head around, esp if mapping sub circuits to additional inputs on PVOutput. Recreating the PVOutput graphs in iotawatt is a little tricky. But all is possible from reading the iotawatt docs, PVOutput docs, and iotawatt forum. I've been meaning to write up a case study there.

          The great thing about iotawatt is it's 14 inputs, high accuracy, and the fact that it's just a CT on a wire. No need for specific API integrations with specific solar inverters etc. And it's at least half the price of inferior commercial systems like solar analytics (which I also have as I got it before I knew about iotawatt).


      And in Sydney, I can recommend Meridian Air Conditioning (I know Raph personally, great guy and great work)


    what's your thoughts?

    I have a lot of them, but I'm not going to waste the good ones here, so let me just say its all about the ambiants.
    No, not the "vibes". Or a hick way of saying 'ambulance'.

    Ambient temperature. The temperature of the air around the heat exchanger (that part of the air conditioner that sits outside. You know, the thing doing the actual work). If you want to run an AC most efficiently, you want to minimise the temperature delta between the temperature the AC is set at, and the heat exchanger.

    Most guides say that an AC will run most efficiently at 7 degrees difference. So if its 35c outside, set it to 28c. If the temperature is 30c, set it to 23c. Sadly there is no mode to do this on most air conditioners, but there really should be.
    The further you go outside that 7 degree range, both the more work the compressor has to do, and the faster the building sucks in heat. You lose on both ends, which is why the power consumption skyrockets.


      is this seven degrees margin proven? I would love to use this as my guidance for slimming my power bill.


        Proven? no idea. Its the figure usually given when you look up air conditioner COP (which is the measure for energy out vs energy in aka efficiency). COP changes based on the temperature delta, and the number quoted on the box is usually only valid up to that 7 degree mark. Past that, and efficiency of operation begins to drop off.


      I think the 7° rule is an old American thing. Farenheit, and only a "duty-cycle approximation" for stop-start ducted building air conditioners?

      My Daikin manuals simply say "Keeping the temperature setting at a moderate level helps save energy.
      • Recommended temperature setting − For cooling: 26.0-28.0°C − For heating: 20.0-24.0°C"

      As a thought experiment, consider:

      • A run of hot days. Your factory or house has heated up to 34°C.

      • It is rare cool morning. 6AM, and still 19°C outside.

      • You turn on your AC. It is old/broken/very inefficient, pumping 34°C refrigerant outside.
        However, because outside is 19°, that is a 14° difference., so your condenser is radiating heat well.

      Now, it doesn't matter what temperature your AC thermostat is set at.
      You could apply the 7° rule and set it at 12if it goes that low, or use common sense and set it at 23.
      As long as it is less than the 34 internal ambient, compressor will pump, condensor will radiate, evaporator will make cooler air inside!
      Now, lets consider the cost…

      1) Say it is an unusually cool day, and only gets up to 27.
      If you are running it for 10 hours, setting at 19 or 20 will use a lot more power than setting at 23.
      (because this broken AC will never get down to 20, so the duty cycle is 100%. At least it might get down to 23 and cutout)
      So much for the 7degree guide.

      2) You get someone to service your 40 year old AC. It now pumps refrigerant at 50psi.
      Outside refrigerant will now be over 40°C, and condenser will radiate heat even better.
      7degree rule now becomes 15degree. Are you going to set it at 19 on a 34degree day?
      No, because the amount of heat you pump, to drop inside from 34 to 19 is nearly double 34 to 24.
      (and the cooler it gets inside, the harder it is for the evaporator to absorb heat)

      3) If, like most of us, you have some sort of inverter-driven compressor, it can do a much better job.
      It starts up slowly (less friction and current drawn), and pumps at low pressure if internal heat load is low.
      Thus, the range of outside to inside temperature differences - that are efficient - are larger. Maybe 2 to 15.
      (also I suspect it can operate at higher maximum outside temp., because it can detect/change vapour point)

      And, because this is OzBargain, lets push this to the ridiculous…

      4) Your cousin from Tasmania moves in. Can't hack the heat. You install a 41kW commercial AC system.
      Its evaporator is twice the surface area, but compressor wastes a bit more power.
      15degree rule becomes 27degree.

      5) You also add insulation. Now, at the end of a hot day/week, the walls radiate less outside heat in.
      27 degree rule becomes 28.5degree. You plant some trees and install a swimming pool beside the outdoor unit.
      This lowers the outside ambient at the heat pump by 2 degrees.
      Most energy efficient difference is now 30.5 degrees. On a 30 degree day, do you set it below zero? :-)


    19 degrees when it's 40+ outside? Just set it to 24-26 and use the Eco mode if you want to save money.

  • +1 vote

    There are only a FEW days of the year that reach into the uncomfortable zone….. why not go to the beach.


    We have 40 square home with ducted, 10 outlets, usually turn on around 12-2pm - 8pm when inside hits about 26-27c, air con on 26c, will cool whole house to 24c which is perfect, if gets too cold will turn up to 27c.
    North coast NSW.


    We have some solar, so yesterday I turned AC on before lunch while panels were generating - to cool the house down and keep it cool.

    1) I think your colleagues who say running 24/7 would be cheaper, are crazy.
    If you know its going to be a hot one, but it is still 22° outside? Open a windows while you can!

    Even with modern "inverter" driven compressors that are pretty good at saving power in all conditions,
    zero power used is better than then using the fan, and ramping the compressor up and down occasionally?

    2) I do know that coming home from work after a very hot day, and pressing the button for instant comfort, is the most expensive way to do it.
    Cheapest way is somewhere in the middle?

    3) Outlander, with his "7 degree" rule, has a point, but it might only be for stop/start type ACs.
    Inverter style split systems probably have a wider range of efficiencies

  • +1 vote

    The best way to save power is to not run it at all. The next best thing it to set it at a higher temp and run it before the heat gets into the house. Yes, cooking from hot will take more energy than keeping it cool. But running 24/7 may not be as efficient as turning it on 2hrs before getting home or running it in a closed house before peak power prices

    Every house is different so you’ll need to learn what temp conditions make your house hottest and preempt that a bit with AC. Use your experience to minimise the use of you are serious about saving power.

    Our house takes a few really hot days to heat up so we hardly use the AC, maybe half a dozen days per season. Same with our previous house. Ceiling fans and open windows, use of blinds to stop the sun entering etc are generally enough, but we are also probably more inclined just to put up with a bit of heat and keep the cool drinks at hand.

    You don’t NEED AC, but it is nice on the hottest/coldest days to take the edge off. I don’t see the point in getting used to having the house at 21 degrees 24/7 but then whinging about the cost of electricity.