Hot water system calculator
Every type of hot water system, ranked for your household and your state's prices. Answer the questions and the whole page recalculates — annual running cost, what it costs to own for ten or twenty years, the emissions, and every rebate you can claim where you live.
Your household
Every system, side by side
The same comparison with the working shown: how much energy each system needs, what it costs to buy and fit, what you can claim back, and where that leaves you over the horizon selected above.
| System | Energy a year | Running cost | Installed | Rebates | 10-year total | CO₂e a year |
|---|---|---|---|---|---|---|
| Solar thermal with gas boost | 2621 MJ | $312 | $6,500 | −$1,130 | $8,488 | 173 kg |
| Heat pump | 1090 kWh | $411 | $4,650 | −$1,130 | $7,628 | 698 kg |
| Gas instantaneous | 8738 MJ | $418 | $2,350 | — | $6,531 | 489 kg |
| Solar thermal with electric boost | 1324 kWh | $498 | $6,500 | −$1,130 | $10,352 | 847 kg |
| Gas storage | 13056 MJ | $516 | $2,400 | — | $7,557 | 673 kg |
| LPG instantaneous | 8738 MJ | $692 | $2,350 | — | $9,267 | 568 kg |
| Electric storage | 2770 kWh | $917 | $1,900 | — | $11,070 | 1773 kg |
| LPG storage | 13056 MJ | $1,004 | $2,400 | — | $12,439 | 791 kg |
Assumes 120 litres of hot water a day for 3 people, heated to 60 °C from an inlet temperature of 18.6 °C, electricity at 33.1c/kWh, gas at 2.72c/MJ with 50% of the daily supply charge attributed to hot water, emissions at 0.64 kg CO₂e per kWh for the New South Wales grid. Rebates are counted at the bottom of their published range and are indicative — the real figure comes from an installer's quote.
Adjust the assumptions
Every figure below starts at your state's published reference rate. Put your own bill in and the whole page follows. Nothing is sent anywhere — these stay in this browser.
Where each number comes from
Open any system to see its full working — the heat actually delivered, the losses, the efficiency or coefficient of performance applied, and the price paid for every unit of energy.
Solar thermal with gas boost — $312 a year
The same collectors with an instantaneous gas burner as the backup. Makes sense where gas is already connected and staying.
| Hot water actually delivered | 2112 kWh of heat a year |
| Supplied by the solar collectors | 70%, leaving 634 kWh to be boosted |
| Efficiency | 87% |
| Fuel used | 2621 MJ a year |
| At | 2.72c per MJ |
| Share of the gas supply charge | $161 a year |
| Electricity for the fan and electronics | $20 a year |
| Total running cost | $312 a year |
Heat pump — $411 a year
A refrigeration circuit run backwards: it moves heat out of the air into the tank rather than making it. Two to three times as efficient as an element, and the only electric system that beats gas on running cost almost everywhere.
| Hot water actually delivered | 2112 kWh of heat a year |
| Nameplate COP | 3.2 |
| Effective COP here | 2.33 — after the climate and in-service derating |
| Standing loss | 183 kWh a year keeping the tank hot |
| Electricity used | 1090 kWh a year |
| At | 33.1c per kWh |
| Total running cost | $411 a year |
Gas instantaneous — $418 a year
Heats water only while a tap is running, so there is no standing loss. That is worth roughly $80 a year against gas storage, and it lasts noticeably longer.
| Hot water actually delivered | 2112 kWh of heat a year |
| Efficiency | 87% |
| Fuel used | 8738 MJ a year |
| At | 2.72c per MJ |
| Share of the gas supply charge | $161 a year |
| Electricity for the fan and electronics | $20 a year |
| Total running cost | $418 a year |
Solar thermal with electric boost — $498 a year
Roof collectors heat the water directly; an element covers the shortfall. Long-lived and cheap to run, but it needs the right roof and the highest outlay in this table.
| Hot water actually delivered | 2112 kWh of heat a year |
| Supplied by the solar collectors | 70%, leaving 634 kWh to be boosted |
| Efficiency | 95% |
| Standing loss | 657 kWh a year keeping the tank hot |
| Electricity used | 1324 kWh a year |
| At | 33.1c per kWh |
| Total running cost | $498 a year |
Gas storage — $516 a year
A tank heated by a gas burner. Keeping 130-odd litres hot around the clock costs about 8 MJ a day whether anyone showers or not.
| Hot water actually delivered | 2112 kWh of heat a year |
| Efficiency | 75% |
| Fuel used | 13056 MJ a year |
| At | 2.72c per MJ |
| Share of the gas supply charge | $161 a year |
| Total running cost | $516 a year |
LPG instantaneous — $692 a year
Common where there is no gas main. Avoiding the standing loss matters more on LPG than on natural gas, because every megajoule costs more.
About 4.0 × 45 kg bottles a year. Bottle rental, if your supplier charges it, is on top.
| Hot water actually delivered | 2112 kWh of heat a year |
| Efficiency | 87% |
| Fuel used | 8738 MJ a year |
| At | 7.69c per MJ |
| Electricity for the fan and electronics | $20 a year |
| Total running cost | $692 a year |
Electric storage — $917 a year
A tank with an electric element in it. The cheapest thing to buy and, on a flat tariff, the dearest thing in this table to run.
| Hot water actually delivered | 2112 kWh of heat a year |
| Efficiency | 95% |
| Standing loss | 548 kWh a year keeping the tank hot |
| Electricity used | 2770 kWh a year |
| At | 33.1c per kWh |
| Total running cost | $917 a year |
LPG storage — $1,004 a year
The same appliance as gas storage, running on bottled gas at roughly three times the price per megajoule.
About 6.0 × 45 kg bottles a year. Bottle rental, if your supplier charges it, is on top.
| Hot water actually delivered | 2112 kWh of heat a year |
| Efficiency | 75% |
| Fuel used | 13056 MJ a year |
| At | 7.69c per MJ |
| Total running cost | $1,004 a year |
Rebates you can claim
A heat pump installed in New South Wales today can draw on 2 schemes. Each is a separate line item on your quote — none of them is inside the price.
Small-scale Technology Certificates (STCs)
$800–$1200 off the quote · $800–$1,200
Administered by Clean Energy Regulator. Official scheme page. Last verified 5 September 2026.
Hot water upgrade incentive
Up to $640 replacing an electric system, up to $330 replacing gas · $330–$640
- Applied as an upfront discount at installation — there is nothing to claim back afterwards.
- A minimum customer co-payment of $220 including GST applies under the ESS Rule that commenced 12 September 2025.
- Systems with a storage volume of 700 L or less need a five-year warranty on both the tank and the heat pump unit.
The incentive is larger for replacing electric than gas, which is the opposite of what most people expect. The scheme pays for energy saved, and an electric storage tank wastes far more of it.
Administered by NSW Climate and Energy Action, under the Energy Savings Scheme. Official scheme page. Last verified 5 September 2026.
Rebate values are indicative and every one of them is shown as a separate line item rather than folded into a price. The figure that counts is the one on an accredited installer's quote. See the full rebate guide for eligibility detail.
How the calculation works
The physics is the easy part and it is the same for every system. Raising water from the inlet temperature to 60 °C takes a fixed amount of heat:
Q = volume × 4.186 kJ/kg·°C × (60 °C − inlet temperature)
That heat requirement is calculated month by month rather than annually, because the water coming into your house is several degrees colder in July than in January, and averaging that away costs a few per cent of accuracy in the southern states.
Worked example
A three-person household in Sydney uses 120 litres a day. Sydney's annual mean cold water inlet temperature is 18.6 °C, so each day's hot water needs:
120 × 4.186 × (60 − 18.6) = 20,796 kJ = 5.78 kWh of heat a day, or about 2,109 kWh a year.
An electric storage tank is about 95% efficient at the element and loses roughly 1.5 kWh a day keeping the tank hot, so it draws 2,109 ÷ 0.95 + 548 = 2,768 kWh a year. At the New South Wales reference rate of 33.1c that is about $916.
A heat pump delivers the same 2,109 kWh of heat at an effective coefficient of performance of 2.33 in Sydney, plus about 0.5 kWh a day of standby: 2,109 ÷ 2.33 + 183 = 1,088 kWh a year, or about $360. The difference — roughly $550 a year — is what a heat pump is actually worth.
What the totals include
The ten and twenty year views add the installed cost, subtract the rebates at the bottom of their published range, and add a replacement where the system will not survive the horizon. They do not apply energy price inflation by default: compounding a guess about future prices makes a calculator look precise rather than making it accurate.
Full detail, including every source and the benchmark this model is validated against, is on the methodology page.
Frequently asked questions
Why does this calculator ask what else I use gas for?
Because the daily supply charge is often a bigger term than the gas itself. A household with gas only for hot water pays the whole connection charge — around $320 a year in New South Wales — to have hot water, and switching away from gas saves all of it. A household that also cooks and heats with gas keeps the connection regardless, so hot water should only carry a share. Getting this wrong moves the answer by hundreds of dollars, and most comparisons ignore it entirely.
Why is the heat pump figure lower than the manufacturer's?
Manufacturers quote the coefficient of performance measured under laboratory conditions at a favourable ambient temperature. That test does not see compressor cycling, standing losses in the tank and pipework, or a household drawing four showers in an hour. Our figures are calibrated so the model reproduces Sustainability Victoria's measured Melbourne running costs, which puts the effective annual figure at a little over 2 against a nameplate of 3.2.
Can I trust the installed cost figures?
Treat them as a starting range, not a quote. They are the midpoint of ranges surveyed across Australian installer pricing in 2026, and the spread within each range is real — whether the new system fits the old one's footprint, and whether a new circuit or gas line has to be run, move the figure by more than a thousand dollars. The running cost figures are far more reliable than the installed cost figures, because physics varies less than plumbers do.
Why does the ranking change when I switch to the ten-year view?
Because the cheapest system to run and the cheapest system to own are frequently different systems. Solar thermal has the lowest running cost almost everywhere in Australia and one of the highest installed costs, so it tends to lead on the annual view and fall back on the ten-year one. That divergence is the single most useful thing this calculator shows, and it is why it defaults to displaying both.
What if my house has no gas and no room on the roof?
Then the comparison is between a heat pump and an electric storage tank, and the heat pump wins on running cost in every state — by roughly $550 a year for a three-person household in Sydney. Set your gas connection to "no gas at all" and your property type to apartment, and the calculator will grey out the options that are not available to you rather than quietly dropping them.