What does a air conditioning (cooling) cost to run?

Most New Zealand 'air conditioning' is just a heat pump running in reverse to cool, so if you run yours in summer this is the row for it. The good news is that it is efficient at it. The same COP advantage that makes heat pumps cheap to heat also makes them cheap to cool, delivering several units of cooling for each unit of electricity.

For most of the country, summers are mild enough that cooling is a minor seasonal cost, not a year-round burden. For the typical NZ home, cooling in summer is far smaller than heating in winter, so if your bill has a nasty spike it is almost always the winter one, not the summer one.

That is changing in the hotter, drier regions. Hawke's Bay, Marlborough, Central Otago and inland Canterbury now see real hot spells where cooling runs for weeks rather than the odd afternoon, and in those places it becomes a genuine seasonal line on the bill worth paying attention to.

The common mistake is the same one people make with heating, just in reverse: setting an extreme temperature and expecting the unit to blast the room cold. That runs the compressor flat out and costs the most. A moderate setting the unit can hold steadily is both cheaper and more comfortable.

The efficient approach is the same as for heating. Set a sensible temperature, around 24-25°C for cooling, use the timer, and close up the house during the heat of the day so you are not fighting the sun. Shade and keeping windows and curtains shut through the hottest hours do a surprising amount of the work for free.

Pairing it with ceiling fans lets you set a higher temperature for the same comfort, which trims the cost further, because moving air makes a room feel several degrees cooler. To see how small the summer cost really is for you, put your cooling hours and region's rate into the calculator and compare it with the same unit's winter heating figure.