A single 40-degree day is manageable, but a run of them is what blows out a summer bill. During a heatwave an air conditioner may battle from midday to midnight, and if the house is poorly sealed it never quite wins, so the compressor runs and runs. Because cooling is already the biggest load in most homes, a week-long heatwave can add more to the quarter than everything else combined.
The goal during a heatwave is not to suffer without cooling. It is to get the most cool for each unit of energy. That comes down to keeping heat out, cooling smart, and lining cooling up with cheaper power where you can.
Keep the heat out before it gets in
The cheapest cooling is the heat you never let inside. Close blinds, curtains and external awnings on the sunny side of the house early, before the day heats up. Glass in direct sun is a major heat source. Keep windows and doors shut through the hottest part of the day to hold the cool air in, then open the house up at night if it cools down outside to flush the heat out.
Seal the gaps. Draughts around doors and windows let cooled air escape and hot air in, making your system work harder for the same comfort. A well-sealed, shaded room holds its cool for hours; a leaky, sun-struck one bleeds it in minutes.
Cool smart during the peak
Set the thermostat to around 24-25°C. Every degree cooler sharply increases energy use, and the difference from 20°C is large.
Cool only the rooms you are using: zone a ducted system, or close doors and run a single split system, rather than cooling the whole house.
Run ceiling or pedestal fans alongside the air conditioner. Moving air feels cooler, so you can set the thermostat a couple of degrees higher for a fraction of the energy.
Pre-cool earlier in the day, especially if you have solar or a cheaper daytime rate, so the house coasts through the expensive late-afternoon peak instead of the system working hardest when power is dearest.
Clean or replace the filter before summer. A clogged filter makes the system work harder and cool less.
Make sure your plan is not making it worse
Heatwave cooling often lands squarely in the time-of-use peak, when power is most expensive. If a big chunk of your cooling happens in that late-afternoon and evening window, your peak rate is doing real damage, and a better-matched plan can soften it.
EnergySorted costs every plan against your real usage, including your peak and off-peak split and any solar feed-in, with no retailer commission. It will show whether shifting to a plan with a lower peak rate, or pre-cooling on a solar-sponge daytime rate, would leave you better off through the hot months.
Frequently asked questions
What thermostat setting saves the most in a heatwave?
Around 24-25°C. Each degree cooler you demand sharply raises energy use, so a setpoint in the mid-20s with a fan running keeps you comfortable for far less than chasing 20°C.
Should I pre-cool the house before a heatwave peak?
Yes, especially if you have solar or a cheaper daytime rate. Cooling the house earlier in the day lets it coast through the expensive late-afternoon peak instead of the compressor working hardest when power is most costly.
Do fans help if I already have air conditioning?
Very much. Moving air makes you feel a few degrees cooler, so you can raise the thermostat setting while staying comfortable, and a fan uses a tiny fraction of the energy an air conditioner does.
Is it cheaper to cool one room or the whole house?
Cooling only the rooms you use is cheaper. Zoning a ducted system, or closing doors and running a single split system, avoids paying to cool space no one is in, a big saving during a long heatwave.
Can my energy plan reduce heatwave cooling costs?
It can reduce the rate you pay for that cooling. If much of it lands in the time-of-use peak, a plan with a lower peak rate or a cheap solar-sponge daytime rate can help. EnergySorted compares the market on your real usage to find the best match.
What else drives up my bill in summer besides the heatwave?
Pool pumps running longer, a second fridge working harder in the heat and more time at home all add up alongside the cooling. Our broader guide on how to cut your summer electricity bill covers the loads worth tackling before and after a hot spell.
Are there rebates or concessions that help during extreme heat?
Many states offer energy concessions and hardship support, and some run life-support or medical-cooling rebates for households that need to run cooling for health reasons. It is worth checking the rebates and concessions you may be entitled to before summer.
What should I do if a heatwave bill leaves me struggling to pay?
Contact your retailer early, because everyone has hardship protections and can offer a payment plan or extension. Our guide on what to do if you cannot afford your electricity bill walks through your options so a hot quarter does not turn into arrears.
Is it better to close the house up or open the windows in a heatwave?
Close up during the hottest part of the day, with blinds and curtains drawn on the sunny side to keep radiant heat out. Then open the house at night if it cools down outside to flush the trapped heat before the next day. Timing the two makes a real difference to how hard the air conditioner works.
Does buying a more efficient air conditioner pay off?
Over a hot climate and years of use, a higher star-rated inverter unit can pay back the difference, because it delivers more cooling per unit of power. If your current system is old, ducted and struggling, running costs alone can justify the upgrade, though sealing and shading the house comes first.
General information only, current at the time of writing. Not financial advice. Rebate schemes and rules
change; always confirm details with your retailer or state government energy site.
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