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Tell us the house and the question
Call or message with the basics - house type, rooms, what you are weighing up. Two minutes of context makes the advice worth having.
Mostly no. Time it to start before you get home instead. Here's the honest maths.
Is It Cheaper to Leave Air Conditioning On All Day?
Running Costs
The Straight Answer
An empty house doesn't need cooling. A timer beats both extremes.
Is it cheaper to leave the aircon running all day? For most Sydney homes, no. Cooling an empty house for eight hours costs more than one hard pull-down when you get home, even though the pull-down uses peak power. The exception is a very well insulated home on a brutal day, where holding temperature is cheap. The smart play that beats both: set the timer to start 30 to 45 minutes before you walk in.
This question gets argued at every Sydney barbecue because both sides are half right. Inverters ARE efficient at holding a steady temperature. But holding 24 degrees in an empty house from 9am to 6pm still costs real money the timer version doesn't. The all-day approach only wins when the house barely leaks heat.
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Quick Numbers
The Maths on a 35 Degree Sydney Day
Typical 5kW living area split, modern inverter.
Left on all day, 9am to 10pm: roughly $8 to $12
Cruising at low power most of the day, but for 13 hours - including 9 hours when nobody is home to enjoy it.
Timer start at 5.30pm, off at 10pm: roughly $4 to $6
One hard hour pulling the room down, then cruising. Half the cost, same comfort from the moment you walk in.
Switch on when you walk in at 6pm: roughly $3.50 to $5.50
Cheapest on paper, but you sweat through the first half hour while it pulls a 30 degree room down. The timer costs cents more and skips that.
Every degree colder: about 10 percent more
Whatever schedule you run, 24 degrees instead of 21 saves real money over a summer. Pair it with a ceiling fan and you won't feel the difference.
Why the All-Day Myth Survives
Because it's half true. Inverter systems hate short cycling - on, off, on, off every twenty minutes genuinely wastes power and wears parts. So "leave it running instead of flicking it on and off constantly" is good advice. Somewhere along the way that turned into "leave it on all day even when you're out", which is not. The compressor holding temperature in an empty house at 2pm is doing real work against the afternoon heat, and you're paying for every minute of it.
The other half-truth is that the startup pull-down is expensive. It is - briefly. An hour of hard running costs less than six hours of moderate running. The maths almost always favours the timer.
Common Factors
What's Actually Gone Wrong
The common questions about leaving it on, answered straight.
When does leaving it on all day make sense?
Well insulated home, someone home most days, or pets that need the heat taken off. Then holding 25 to 26 through the day and dropping it in the evening is reasonable.
Does turning it on and off damage the system?
Normal daily on and off, no. Constant short cycling every few minutes, yes – that usually means an oversized unit or a faulty sensor, and it’s worth getting checked.
What about ducted - same logic?
Same logic, bigger numbers. Ducted holding a whole empty house all day is the most expensive version of the myth. Zone it and time it – bedrooms cool from 8pm, living areas from before you’re home.
Does Wi-Fi control help?
It’s the modern timer. Flick it on from the car twenty minutes out and you get the timer’s savings with none of the planning. Most current systems have an app or can be retrofitted.
The Setup That Actually Saves Money
Set 24 degrees, not 21. Use the timer or the app so the system starts shortly before you're home. Run ceiling fans with it. Keep filters washed so the system isn't fighting itself - a choked filter quietly adds 20 percent to every hour of running. And if you're on ducted, make sure zoning is set up properly so you're not cooling guest rooms nobody has slept in since Easter. More on bills on our high power bills page, and real hourly numbers on our running costs page.
Diagnosis Process
If It's Expensive No Matter What You Do
A system that costs a fortune on any schedule has something wrong - dirty coils, low gas, tired compressor or the wrong size for the room. A $250 plus GST service finds it. Since 1991, 4.8 stars from 280+ reviews. (02) 9820 4500.
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We measure, not guess
Sizes come from volume and glass, verdicts come from testing. No brochure numbers, no sight-unseen quotes.
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You get the numbers and the options
Real installed prices, repair-versus-replace maths, and the trade-offs in plain English. You decide with the facts in front of you.
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Fixed quote before any work
The price agreed is the price paid, and installs and repair parts carry lifetime workmanship warranty.
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Fixed quote before we fix
Once we know the fault, we quote the repair before we start. You decide. Most jobs get done same visit.
Real Jobs
Thinking of Upgrading the Old Unit?
If you’re running a 15-year-old system on any schedule, a modern inverter will beat it by 30 to 50 percent on power. Check real installed prices on the pricing guide and size it right with the sizing calculator.
Straight Answers, Local Crew
We'll tell you the cheap fix before the expensive one, every time. Family run since 1991, fixed pricing, licence 273312C, ARCtick AU13058. Give us a bell on (02) 9820 4500.
Related reading: real hourly running costs and how servicing keeps those costs down.
FAQ's
Aircon All Day - Common Questions
For most homes, no. Cooling an empty house costs more than a timed start before you get home. The exception is a very well insulated house on an extreme day.
Yes, the initial pull-down runs the compressor hard – but only for the first 30 to 60 minutes. That burst costs less than holding temperature in an empty house for hours.
Timer or app start 30 minutes before you’re home, 24 degrees not 21, ceiling fans running with it, clean filters, and zoning if you’ve got ducted. Each one is a real saving and they stack.
On a hot day, yes – but set it to 26 or 27, not 22. That takes the danger out of the heat at a fraction of the cost of full comfort cooling.
Short cycling usually means the unit is oversized for the room or a sensor is faulty. It wastes power and wears the compressor – worth a proper look.
Jay’s Air. Service is $250 plus GST and covers the usual cost culprits. Family run since 1991, fixed pricing, 4.8 stars from 280+ reviews. (02) 9820 4500.