As a modelled estimate for a family of four: about $1,450 a year for an electric storage tank on a general tariff, about $900 in gas for a gas storage system, and about $410 for a heat pump. The heat pump saves roughly $600 to $1,000 a year against electric. Here is the arithmetic, so you can redo it with your own tariff.
How much heat a household uses
Assume four people using about 50 litres of 60°C water each a day, heated from 15°C. Heating 200 litres by 45°C takes 200 x 45 x 4.186 kJ, about 37.7 MJ or 10.5 kWh a day. Add about 1.5 kWh a day of heat lost from the tank and pipes: 12 kWh a day, or about 4,380 kWh of heat a year.
That is our modelled assumption, not a measurement of your home. A household that showers longer, or lives somewhere colder, uses more.
The three systems side by side
| System | Energy bought a year | Cost a year |
|---|---|---|
| Electric storage tank (all heat from the element) | 4,380 kWh at 33 c/kWh | about $1,450 |
| Electric storage on off-peak | 4,380 kWh at 20 c/kWh | about $880 |
| Gas storage (70% efficient) | 22,500 MJ at 4 c/MJ | about $900 + supply charge |
| Heat pump, COP 3.5 over the year | 1,250 kWh at 33 c/kWh | about $410 |
| Heat pump on off-peak | 1,250 kWh at 20 c/kWh | about $250 |
Modelled estimates. Tariffs are assumptions: 33 c/kWh general, 20 c/kWh off-peak, 4 c/MJ gas. A COP of 3.5 is a cautious year-round figure; the SHP-O is rated 4.2 under test conditions (EN 16147, A20). If hot water is the only gas appliance, the gas daily supply charge (several hundred dollars a year) comes on top, and switching saves it.
Redo it with your tariff
Find the price per kWh on your electricity bill. Multiply it by 4,380 for an electric tank, or by 1,250 for a heat pump. The difference is your yearly saving, before the rebate and before any solar.
If you have solar panels
A heat pump draws about half a kilowatt while it runs, so a modest solar system covers it easily in the middle of the day. Set it to run on a daytime timer, or use the solar input on the WWK and SHP-O, and most of that 1,250 kWh comes from your roof instead of the grid.