What does a heat pump (heating) cost to run?

The heat pump is the most important appliance in most New Zealand homes. It is the cheapest way to heat, and if you let it run hard, a fast way to push up a winter bill. The reason it matters so much is that space heating is around 30% of a typical NZ home's energy use, and how you produce that heat decides most of your winter cost.

The number on the box, '2.5 kW' or '6 kW', is heat output, not electricity used. A well-running unit delivers three to four units of heat for every unit of power it draws. That multiplier, the COP, is why it beats any plug-in heater. A resistive heater is stuck at roughly one unit of heat per unit of power, so for the same warmth it costs three to four times as much to run.

The marketed efficiency assumes mild conditions. On a frosty NZ morning at 0-5°C, real-world performance drops to a COP closer to 2.5, and the unit also burns energy defrosting its outdoor coil. It is still well ahead of a plug-in heater on that cold morning, just not as far ahead as the brochure suggests. The calculator uses that real-world winter figure, so the number you see already accounts for a Canterbury frost.

Two mistakes are common. One is buying too small a unit for the room, so it runs flat out and never reaches temperature, which is both uncomfortable and expensive. The other is treating it like a campfire, blasting 28°C for an hour then switching off, then doing it again an hour later when the room has gone cold.

Set it around 18-20°C and let it hold a steady temperature. A heat pump is at its most efficient ticking over gently to maintain a room, not roaring back from cold. Use the timer so it warms the room before you need it rather than chasing the temperature all evening, and clean the filters, because a clogged filter makes the unit work harder for less heat.

The savings live in those settings. Put your own region's rate and the hours you actually run the unit into the calculator, and you can see the gap between running it sensibly and running it hard, and how it stacks up against the plug-in heater it might be replacing.