New Zealand's Specialist Hot Water Heat Pump Installer
Serving Wellington, Hutt Valley, Wairarapa, Kapiti, Horowhenua & Manawatu
Serving Wellington, Hutt Valley, Wairarapa, Kapiti, Horowhenua & Manawatu
One brand claims a COP of 4.5. Another claims 5.0. Yet another boasts 5.2.
If you’ve been researching hot water heat pumps, you’ve probably come across COP figures and wondered what a good coefficient of performance to look for is. Naturally, it’s tempting to assume the highest number must be the best. But it’s not quite that simple.
The problem isn’t that manufacturers are lying; it’s that not all COP figures are measured under the same conditions, which means comparing them side by side is rarely a fair fight.
Here’s how the coefficient of performance actually works, why the published figures can mislead you, and what you need to know so you end up with a system that performs all year-round, instead of just looking good in a brochure.
Key Takeaways:
COP stands for coefficient of performance. It’s a ratio that measures how much useful heat a system delivers compared to the electrical energy it consumes.
The heat pump coefficient of performance formula is simple:
COP = Heat Output (kW) ÷ Electrical Input (kW)
A standard electric cylinder converts electricity directly into heat, so for every 1kW in, you get roughly 1kW out and a COP of about 1.0.
A hot water heat pump works differently. Instead of generating heat from scratch, it pulls thermal energy from the surrounding air and transfers it into your water, using the compressor as the engine. Because it moves heat rather than creates it, far less electricity is required to achieve the same result. If the unit uses 1kW of electricity and produces 4kW of heat, its COP is 4.0.
Now for the part the spec sheets don’t tell you.
Knowing how to find the coefficient of performance on a product listing is easy: look for “COP” on the manufacturer’s datasheet, usually alongside a test temperature, such as “COP 4.2 @ 20°C.” The catch is in those last few characters.
One of the biggest challenges facing consumers is that there is currently no universally adopted way to measure or report COP figures in New Zealand. Not all published COP figures are measured under the same conditions, making direct comparisons unreliable.
Different manufacturers may test using different:
Some figures are peak efficiencies achieved under ideal laboratory conditions. Others are averages. Some are based on overseas standards, while others are based on different testing methods altogether.
This means two products with similar advertised COP figures may perform very differently in the real world. And a product with a lower published COP may actually use less electricity over the course of a year.
Imagine buying a car based solely on a brochure claiming 4 litres per 100km. That sounds great. But was that measured on a flat road, no traffic, no air conditioning, and a tailwind?
The same principle applies to hot water heat pumps. The heat pump coefficient of performance formula is straightforward. What isn’t straightforward is the set of conditions manufacturers use when they run that calculation. The number itself isn’t wrong. It’s just only part of the story.
Every COP figure is measured at a specific ambient air temperature. The warmer the air, the easier it is for the heat pump to extract thermal energy, and the higher the COP. That’s why some manufacturers test at 20°C or even 35°C. Those figures look great in a brochure, but they’re also largely irrelevant if you live in Wellington, the Manawatu, or anywhere that sees a proper winter.
A heat pump might show a high COP at a 35°C test ambient, but the same unit operating at a more realistic NZ winter temperature of 7°C can look quite different in terms of output and efficiency. Wellington averages 6–8°C on winter mornings. Palmerston North can be even colder. A system optimised for mild climates will work harder, use more electricity, and deliver a lower real-world COP than the datasheet suggests.
It’s important to note that there’s no universal “good” COP without context because the test conditions strongly influence the result. A COP of 4.8 tested at 35°C ambient air may serve you worse through winter than a 3.8 tested at 7°C.
Geography matters too. A heat pump installed in Auckland will have a higher real-world COP than the same model installed in Christchurch, simply because the average air temperature is higher and the system doesn’t have to work as hard to transfer heat to the water.
At A2W, we size and specify our Wellington and Manawatu installations against local winter temperatures, not the conditions that make a spec sheet look its best.
Real homes aren’t laboratories. Efficiency changes constantly, which is why chasing the highest advertised COP figure can lead people to focus on the wrong thing.
As installers, we’re far more interested in how a system performs over an entire year than what it can achieve under ideal conditions for a few hours.
The question shouldn’t be: “Which hot water heat pump has the highest COP?”
It should be: “Which system will provide reliable hot water and minimise my power bill over the long term?”
Those are two very different questions.
A heat pump that saves a few extra dollars a year but suffers reliability issues or poor support quickly becomes an expensive investment.
Efficiency is important. Reliability is important. Correct sizing is important. Quality installation is important. And perhaps most importantly, choosing a system that suits the way your household actually uses hot water is important.
COP is a useful number, but it’s only one piece of the puzzle. Without a consistent standard for measuring and reporting efficiency in New Zealand, comparing brochure figures alone will never tell you the full story. What will is a system sized, specified, and installed for the way your household actually uses hot water.
At A2W, COP isn’t just a marketing number to us; it’s a core part of how we size and recommend systems. If you’re unsure how to find coefficient of performance figures that actually reflect your home’s conditions, that’s exactly what we’re here for.
When we assess a home, here’s what we’re actually looking at:
Not sure what any of this means for your home? Tell us about your household, and we’ll show you the real numbers and a fixed-price quote with nothing hidden.
At A2W, we specialise in hot water heat pumps for homes in Wellington and Manawatu, and we’ll never recommend a system based on a brochure COP figure alone.
We spec for your climate, household, and your budget. Get in touch with our team for a fixed-price quote with no hidden costs on a system that makes sense for your home today.
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