Heatwaves are where the bill damage happens
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 16,000+ plans 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.