Envirosun HP Plus M9

STCs are calculated by the government from tested performance under the Australian Standard: the more energy a heat pump saves against an electric tank, the more it earns. It's the one efficiency figure every brand is measured the same way on.

Rule: STCs for a Zone 3 installation on the Clean Energy Regulator register, best model in the series: 26 scores 0, 34 scores 10.

Tanks and heat pump units are covered for different terms. Both matter: a tank leak or a dead compressor is the expensive failure.

Rule: Average of the cylinder and the refrigeration-system parts warranty, in years: 3 scores 0, 10 or more scores 10.

Below its limit, a heat pump hands over to the electric element and costs as much to run as an old tank.

Rule: Lowest outdoor temperature the heat pump runs at without the element: 0°C scores 0, -10°C or colder scores 10.

Heat pumps often sit near bedrooms or a neighbour's window.

Rule: Sound pressure at 1 metre: 55 dB(A) scores 0, 37 dB(A) scores 10. Figures quoted at 2 metres have 6 dB added.

Natural refrigerants have almost no climate impact and are what new systems are moving to; older synthetic ones are being phased down.

Rule: Natural refrigerants R744 (CO2) and R290 score 10; R513a 5; R32 4; R134a 2.