Is it cheaper to leave the air conditioner on all day?

For most homes, running the air conditioner all day while nobody's there costs more than switching it on shortly before you get home. That holds for modern inverter systems too, summer and winter. Heat moves through the roof, walls and glass faster the bigger the gap between inside and outside, so holding a cool or warm house against the weather for eight empty hours means paying for that gap the whole time.

Let the house drift while you're out and the gap shrinks, so less heat moves through the building. The system then has some catching up to do, but in most homes the catch-up uses less energy than holding the temperature all day.

The idea isn't baseless. Inverter units are most efficient running steadily at low output and less efficient flat out, so a unit switched on into a hot house works hard for a while, which feels expensive. And on extreme days, a house that has been left to bake can be more than the system can pull down by bedtime.

Those points narrow the gap; they rarely reverse it. The answer to them is timing rather than all-day running: start the system early enough that it's working at a sensible pace before the house is fully heated through.

On ordinary warm days, use the timer or app to start cooling half an hour to an hour before you get home, at 24 to 26 °C. The Victorian Government says each degree beyond its suggested ranges adds around 10% to running costs, so the set point matters more than the start time.

Hot northerly days are the exception. Laverton averages 9.4 days a year at or above 35 °C, mostly December to February. On those days, start well before the afternoon peak, or hold a moderate set point from late morning, so the house never heats past what the system can handle. Close the blinds and windows before you leave.

Victorians typically use more energy for heating than cooling, so winter habits matter more to the bill. Let the house cool while you're out and overnight, and set the timer to start heating before you wake or arrive, at 18 to 20 °C.

Laverton's July mean minimum is 5.2 °C, and in the cold, frosting conditions below 5.5 °C, a reverse-cycle unit pauses now and then to defrost its outdoor coil. Give it a longer lead time on frosty mornings so the house is warm when you get up, rather than still catching up.

Your house and routine can tip the balance one way or the other.

Age matters too. The Victorian Government says a reverse-cycle system more than 15 years old will typically use more than three times the energy of a modern one, so with a unit that old, how you run it matters less than whether it's time to replace it.

The easiest way to get this right is a schedule. Most current units have a built-in timer, and the maker's Wi-Fi adapter or controller lets you adjust it from your phone when plans change. We can set up schedules room by room, so the house is comfortable when you arrive without anyone remembering to press a button.

Keep the filters clean as well. Dirty filters cut airflow and efficiency, which costs you whichever approach you choose.