How hot-water heat pumps work (and why they’re so efficient)

19 July 2025

Hot water quietly eats up around 30% of household energy in New Zealand homes. That’s the shower, dishes, washing, and sometimes even heating. It’s a big chunk of your bill.

The good news? You can shrink it without taking cold showers.

Fridge in reverse

Think about your fridge. It pulls heat out of your food and pushes it into the room.

A hot-water heat pump does the same trick in reverse. It pulls low-grade heat from the air outside and moves it into your hot-water cylinder.

It uses:

·       a refrigerant that boils at very low temperatures,

·       a compressor that “squeezes” that heat, and

·       a condenser coil to transfer it into your tank.

No magic—just physics.

Why moving heat is smarter than making it

An old-style electric element makes heat from scratch. That’s always one unit of electricity for one unit of heat.

A heat pump works differently. Engineers call it the coefficient of performance (COP). In plain English: how many units of heat you get for every unit of electricity you put in.

·       A good unit in NZ has a COP of 3 or better at 7 °C.

·       That means 1 kWh of electricity gives you about 3 kWh of heat.

·       That’s why households often see 60–75% cuts in hot water power use.

Think of it like carrying groceries. Making heat is like growing apples from seed. Moving heat is like carrying apples already in the bag. Much less work.

(See EECA’s full explainer here).

What to ask your installer

Not all systems are equal. Before you choose, check:

·       COP at 7 °C (a real winter morning in most NZ towns).

·       Noise rating if the unit sits near a bedroom or fence.

·       Recovery time—how fast the unit can re-heat your chosen tank size.

Why now?

Power prices are rising. Old cylinders are often on their last legs. And with new installer guidance from EECA, you can trust today’s systems to deliver reliable savings.

The bottom line

If you’re still heating water with an old cylinder, a hot-water heat pump is one of the fastest, easiest ways to cut your bills and carbon.

👉 Book a free assessment with us today.