Hot Water Heat Pumps: What Every Homeowner Should Know Before Buying

Thinking about a hot water heat pump?

If you’re considering replacing your hot water system, you’ve probably heard the term “hot water heat pump” thrown around. But what actually is it, and why are they becoming so popular? Before you buy, there are a few things we think every homeowner should know.

In this guide, you'll learn:

✔ Why a slightly larger cylinder doesn't necessarily cost much more to run.

✔ Why advertised COP figures don't always reflect real-world performance.

✔ How to get more value from solar by using your hot water cylinder as a thermal battery.

✔ What really matters when choosing a hot water heat pump, not just the purchase price.

✔ How to avoid common mistakes that could cost you money or leave you disappointed.

If you're spending thousands of dollars on a new hot water system, it's worth taking a few minutes to understand what you're buying.

First things first: What is a hot water heat pump?

Think of your fridge, but running in reverse. A fridge pulls heat out of its interior and dumps it into the room. A hot water heat pump does the opposite, it pulls heat out of the surrounding air and transfers it into the water sitting in your cylinder.

A standard electric hot water cylinder heats water the old fashioned way, using an element (a bit like an oversized jug element) that converts electricity directly into heat. A heat pump instead uses a small amount of electricity to capture heat from the surrounding air and transfer it into your water, it doesn’t have to create all that heat itself.

That difference matters a lot for your power bill. Because it’s moving heat rather than generating it, a quality hot water heat pump typically uses 60–75% less electricity than a conventional electric cylinder, under the right conditions.

There are two common setups:

Integrated system – the heat pump unit sits right on top of the cylinder.
Split system – the heat pump sits separately and is connected to the cylinder via pipework.

Bigger isn’t always better… but neither is smaller

One of the biggest misconceptions is that the smallest cylinder that meets your household’s needs is automatically the best.

Most New Zealand sizing guides are built to ensure the average household has just enough hot water. They’re a good starting point, but they’re based on minimum acceptable capacity, not necessarily what’s best for long term comfort and system performance.

In many homes, going slightly larger than the minimum has real advantages:

More buffer, less strain. A larger cylinder stores more hot water, giving the heat pump more time to recover after showers and reducing how often the electric backup element needs to kick in during high demand.

Room to grow. It also gives you headroom if the number of people in the house increases, whether it’s guests staying from time to time or additional people moving into the house.

Gentler on the equipment. A bigger tank lets the compressor run longer, steadier cycles instead of starting and stopping frequently. Like most mechanical equipment, compressors generally prefer fewer starts and longer, consistent runs, which can mean less wear over time.

The trade off is a very small increase in standing heat loss, simply because a bigger tank has more surface area. Thanks to modern insulation, this usually only adds about $8–$10 a year to your power bill, a small price for better hot water availability and a system potentially lasting longer.

Don’t judge a heat pump by its COP alone

You’ll often see manufacturers advertise a COP (Coefficient of Performance). A COP of 4 means the heat pump produces roughly four units of heat for every one unit of electricity it uses, so the higher the number, the more efficient it sounds.

That’s an important figure, but it’s not the whole story.

Many advertised COP numbers are measured under ideal laboratory conditions at a single point in time, usually in warm ambient and cold water inlet temperatures, often also with a lower hot water set point. In everyday New Zealand conditions, performance shifts with the weather, installation quality, and how the system is used.

This is where seasonal COP is worth asking about. Rather than one lab figure, seasonal COP reflects the heat pump’s average efficiency across a full year, accounting for cold winter mornings, mild autumn days, and everything in between. Because air temperature directly affects how easily the heat pump can extract heat, real world seasonal COP is almost always lower than a COP recorded at a point in time, though a well designed system will still perform solidly through winter.

Asking about seasonal COP, rather than just the peak number, gives you a much more accurate picture of what to expect on your power bill across the year. That’s why we encourage customers to look beyond the headline figure. Reliability, local support, spare parts availability, and proven long term performance matter just as much as the advertised efficiency.

Hot water heat pumps and solar: A natural pairing

If you already have solar panels, or you’re thinking about getting them, a hot water heat pump can be one of the smartest ways to make the most of that investment.

Here’s the simple version: hot water stored in a well insulated cylinder is essentially stored energy, just in the form of heat instead of electricity. In that sense, your hot water cylinder can act a bit like a thermal battery.

Solar panels generate the most power in the middle of the day, when many households aren’t home to use it. Rather than exporting that excess power to the grid for a low return, you can use it to run your heat pump and heat your water during daylight hours. That hot water then sits ready in the cylinder, insulated and available, whether you use it that evening or the next morning.

Compared to a standard electric element, a heat pump makes this pairing even more effective because it needs so much less electricity to heat the same amount of water. Your solar generation goes a lot further. Some systems can even be set up to prioritise heating during your solar production window, so you’re using your own power rather than paying for electricity from the grid.

It’s not a substitute for a home battery in every sense (it only stores energy as hot water, not as electricity you can run appliances on), but for the cost, it’s one of the most efficient ways to put spare solar power to good use.

Looking beyond the payback period

Many people focus purely on return on investment when comparing hot water systems. Running costs matter, of course, but it’s worth remembering that electricity prices have steadily risen over time. As power prices climb, the savings from an efficient good quality heat pump only grow larger compared with a conventional electric cylinder.

The cheapest system upfront isn’t always the cheapest system to own over the next 10–15 years.

Why we recommend trusted brands

Right now the market is seeing more low cost imported hot water heat pumps than ever before. Some may perform well, but others come with limited technical support, uncertain spare parts availability, and very little local backup if something goes wrong, whether that’s during the warranty period or years down the track.

A hot water system is one of the hardest working appliances in your home. That’s why we choose to install brands with proven reliability, strong New Zealand support networks, and local technical assistance. It gives our customers confidence that help and parts will still be there long after installation.

Our philosophy

Our goal isn’t to sell you the biggest system, or the most expensive one. It’s to recommend the right system for your household, one that gives you reliable hot water, excellent efficiency, and long term value.

If you’re unsure what size or brand is right for your home, we’re always happy to have a chat and talk you through the options.