Hot water heat pump vs electric cylinder: NZ running cost
Hot water is the quiet heavyweight of the NZ power bill. Heating gets the headlines, but water heating runs 25-30% of the average household's electricity use - and unlike a heat pump on the wall, the cylinder in the cupboard does its work invisibly. Most people never think about it until it dies.
When it does die, you face a choice that sets a chunk of your bill for the next 15 years: replace it with another standard electric cylinder, or step up to a hot-water heat pump. The difference in running cost is large enough to be worth understanding before the plumber turns up.
How each one actually works
A standard electric cylinder is resistive: a heating element, typically 3,000 watts, sits in the tank and turns electricity straight into heat. It is 100% efficient in the narrow sense that every watt becomes heat in the water - but that is also its ceiling. One unit of electricity buys you exactly one unit of heat. There is no clever trick.
A hot-water heat pump does the same job the way a fridge works in reverse. A compressor of around 750 watts pulls warmth out of the surrounding air and pumps it into the tank. Because it is moving existing heat rather than creating it, one unit of electricity delivers around three units of heat. That ratio is the coefficient of performance, or COP. A COP of 3 means the unit is, in effect, 300% efficient. This is the same physics that makes a heating heat pump so much cheaper to run than a plug-in bar heater - just applied to your water instead of your air.
The annual cost maths
Let's do real numbers at the national average of 42c/kWh. A water-heating workload is usually quoted in kilowatt-hours per year because it depends on how much hot water you draw, not on a tidy hours-per-day figure. Two households:
- 2-person home: roughly 2,000 kWh/year of delivered hot-water heat
- 4-person home: roughly 3,500 kWh/year of delivered hot-water heat
For the cylinder, delivered heat equals electricity used (efficiency 1.0). For the heat pump, electricity used is the delivered heat divided by the COP - we'll use a realistic year-round average of COP 3.0.
| Household | Cylinder kWh | Cylinder $/yr | Heat pump kWh | Heat pump $/yr |
|---|---|---|---|---|
| 2-person | 2,000 | $840 | 667 | $280 |
| 4-person | 3,500 | $1,470 | 1,167 | $490 |
The maths behind the 4-person cylinder row: 3,500 kWh × $0.42 = $1,470 a year. The heat pump delivers the same 3,500 kWh of heat but only draws 3,500 ÷ 3.0 = 1,167 kWh, so 1,167 × $0.42 = $490. That's a saving of $980 a year for the bigger household, and around $560 for the two-person one - every year, for the life of the unit.
Upfront cost and payback
None of that is free. A like-for-like replacement cylinder runs about $1,500-$2,500 installed. A hot-water heat pump is $3,500-$5,000 installed, partly because the unit costs more and partly because some retrofits need new pipework or a sensible spot to put the noisy bit.
So you are spending roughly $2,000-$2,500 extra upfront to save $560-$980 a year. That is a payback of around three to six years, after which it is money in your pocket for another decade. The economics are best of all when your old cylinder has just failed - you were going to spend the cylinder money anyway, so only the gap is the real "investment".
The cold-weather catch
Here's the catch: a heat pump's efficiency depends on the temperature of the air it is pulling heat from. On a mild Auckland afternoon it might run at COP 3.5+. On a frosty Invercargill or Dunedin morning, that can sag to 2.2-2.5, and most units fall back to a built-in resistive element when it is really cold - at which point, briefly, they are no better than a plain cylinder. That is why we used a conservative year-round COP of 3.0 instead of the glossy brochure figure.
It still wins over a full year everywhere in NZ. But a household in Christchurch or further south should expect a slightly smaller annual saving than a home in Auckland, and should pay attention to where the unit is sited.
Noise, siting and the practical stuff
A hot-water heat pump has a compressor and a fan, so it makes noise - somewhere between a fridge and a quiet outdoor heat pump. That rules out putting it right outside a bedroom window or a neighbour's. It also needs a reasonable volume of air to draw from, so a sealed cupboard is out unless it is a split system. Integrated units (tank and compressor in one) are simplest; split units (compressor outside, tank inside) are quieter indoors but cost more to install.
Who should switch - and who shouldn't
- Switch if: your cylinder is on its last legs, you have a sensible outdoor or garage spot for the unit, and you live anywhere from Auckland to Canterbury where winters are moderate.
- Probably switch if: you are in the deep south but have a sheltered, slightly warmer install location (a garage, not an exposed south wall).
- Hold off if: your current cylinder is only a few years old and working fine - the payback clock only starts when you would otherwise be buying a new cylinder anyway.
- Look elsewhere if: you have rooftop solar and a controllable cylinder. Sometimes a plain cylinder soaking up free midday solar beats a heat pump running on grid power - the maths flips.
Work out your own number
Your real figure depends on your region's rate and how much hot water your household actually uses. Plug both into the NZ Power Bill Calculator to see the cylinder-versus-heat-pump gap for your own home, then compare it against the other big-ticket items on your bill.
Related guides
- The most expensive appliances in NZ homes - where hot water ranks
- Average NZ power bill by region (2026) - put the saving in context
- Day/night power plans - heating water off-peak is another lever