What we found

We tested our heat-pump water heaters (HPWHs) against a new standard A-grade electric hot-water cylinder.

Performance

There was a big difference in performance across the five models we tested. We expected them to become less efficient as the air temperature dropped - but some were much worse than others. At 15°C all were more efficient than the standard electric cylinder, but by the time the temperature had dropped to 2°C only the Econergy, Quantum and Stiebel were still significantly more efficient.

Integral models
The performances of the Quantum and the Stiebel were similar and both would give useful energy savings in all but the coldest climate zones. The Rheem performed well enough at 15°C but was poor at our two lower test temperatures. It would only be of benefit if installed in a warm climate zone.

Split models
The star of our test was the Econergy, which blitzed its competitors. At all test temperatures it was at least three times more efficient than a standard electric cylinder - an outstanding result for this locally designed and manufactured product. We recommend the Econergy for all but the coldest climate zones. Its manufacturer told us that in regions like central Otago (where winter temperatures are often sub-zero) an electric booster element could be required.

The National performed quite poorly. At 15°C it was 50 percent more efficient than a standard electric cylinder; at 7°C it was on par; at 2°C it was only half as efficient. The National's distributor said its poor performance was because of refrigerant leaking from the heat exchanger during testing. It now has a new design of heat exchanger that should solve the problem.

Noise

These units make a sound similar to a fridge - but louder. They're not particularly loud, but could become annoying at night when background noise levels are low. You probably wouldn't want to locate any of the units outside a bedroom window.

The noise levels we measured ranged from just under 58dBA to 62dBA. The Stiebel was the noisiest initially at over 69dBA; but the distributor then supplied us with an add-on noise hood that dropped the level to just under 62dBA. As a comparison, normal conversation levels are around 60 to 65dBA.

Ripple control

Most electric hot-water cylinders are connected by ripple control, which gives lines companies the ability to switch off water heating at times of high load. For this controlled supply, you generally pay less than the normal electricity rate.

If you use ripple-control electricity for your hot-water system (to find out check your electricity bills or ask your electrician), make sure you look at HPWHs that work with it. Don't worry if you don't have ripple control, you'll still make the same percentage savings on your water-heating bill by changing to a HPWH.

The National was the only model that couldn't be connected to a ripple supply: it lost settings when the power dropped off.