STCs for heat pumps and solar hot water

How many STCs a hot water system gets

Three things set the number, and none of them is the size of the tank:

1. The model. The regulator's register of solar water heaters lists every eligible heat pump and solar water heater model with the certificates it earns in each hot water zone over a system life of 10 years. That figure is the model's STC rating.
2. The hot water zone of the install postcode. Heat pumps have five zones and solar water heaters four, from the regulator's own postcode table, and they are not the panel zones.
3. The install year. Since 2022 the 10-year figure is multiplied by a factor that falls by 0.1 a year: 0.5 for 2026, down to 0.1 in 2030. The result is rounded down to a whole certificate.

So a heat pump the register rates at 31 in zone 3, installed in 2026, gets 31 x 0.5 = 15.5, rounded down to 15 STCs, or $615 at $41 per STC. The same model in zone 4, rated 33, gets 16. The regulator says it will not accept a count worked out any other way.

The quickest check is the regulator's own solar water heater STC calculator in the REC Registry: brand, model, expected install date and postcode in, system type, zone and certificates out. It calls itself an approximate guide, and the register and the install date decide the claim. Our STC calculator covers panels and batteries only.

Why the count comes from the model, not a formula

Panels have a formula, kW x zone rating x deeming years, because a kilowatt of panels produces much the same anywhere in a zone. A water heater has no rating that says how much electricity it saves. That depends on the collector or the compressor, the tank and its insulation, the booster, the hot water load and the climate. So the regulator rates each model instead.

The regulations set the count as the difference, over the system's life, between the energy the system uses (other than sunlight and heat from the air) and the electricity an equivalent electric water heater would use for the same hot water load. A 2016 determination fixes the method: each model is simulated in each climate zone under the Australian standard for heated water systems, AS/NZS 4234:2008, the displaced energy in megawatt hours a year is multiplied by 10, and the result is rounded down. One STC is one megawatt hour, so a heat pump rated 31 in zone 3 is modelled to displace 31 MWh of electricity over 10 years there. The regulator publishes that figure on the register and applies the install year's factor on top.

Four things follow for anyone quoting a job:

• Two heat pumps with the same tank can earn different numbers. A more efficient compressor or a better insulated tank displaces more. On the register as read on 4 November 2026, the 1,178 heat pump entries ran from 13 to 49 certificates in zone 3, and half of them sat between 29 and 32.
• Register names differ from marketing names. The regulator's advice is to ask the manufacturer or retailer for the name used on the register.
• The system must be installed exactly as listed. A missed or changed component makes it ineligible.
• Every entry has dates. Eligible from is the model's certification date to AS/NZS 2712:2007, and eligible to is the certification's expiry or 31 December 2030, whichever comes first. A model that is not rated for a zone has no figure there, and nothing to claim there.

Hot water zones are not the panel zones

The regulator's postcode table for hot water (version 3, 1 January 2020) sets five zones for heat pumps and four for solar water heaters, by postcode range, on climate and solar radiation. The panel zones come from a different instrument that ranks places by solar output, with zone 1 the sunniest. The heat pump zones follow the heat pump climate zones of the standard, and zone 5 takes in the coldest places, such as Canberra, Ballarat and Hobart. The same postcode can sit in three different zones:

The direction matters as much as the number. A heat pump's register figures generally rise from zone 1 to zones 4 and 5, because the baseline is the electricity an equivalent electric heater would use, and heating colder water takes more of it: the Rheem 551270 is rated 26 in Darwin's zone 1 and 33 in Hobart's zone 5. A solar water heater's figures peak in zone 2, the dry inland. So the places that earn the fewest panel STCs can earn the most heat pump STCs. Quote from the hot water table, not from the panel zone in your head.

Models from the register

Entries copied from the register (version 58, published 10 August 2026) on 4 November 2026. The figures are the 10-year rating for an installation in each zone, before the factor for the install year: multiply by 0.5 and round down for a 2026 install. They are the regulator's figures, not ours, and the register changes as models are added, re-rated and removed, so check it before you quote.

Heat pumps, five zones

The Stiebel Eltron entry is eligible from 28 October 2025 to 31 December 2030, and the register shows no figure for it in zone 5. An earlier entry for the same model, eligible to 27 October 2025, carried different figures: the dates on the register matter.

Solar water heaters, four zones

The factor by install year

Hot water systems lose a year of certificates each January, like panels, and the factor falls with it. For the Rheem 551270, rated 31 in zone 3 and 33 in zone 4:

Dollar figures use today's price for comparison. The STC price on a later date may differ.

The install date is the day the system is first able to produce and deliver hot water heated by solar energy, in the words of the regulations, and the regulator's guideline puts it at the day the final installer finishes the job. A job finished in late December 2026 earns at 0.5; the same job in January 2027 earns at 0.4, which is 15 against 12 certificates in zone 3, or $123 less at $41. Certificates must be created within 12 months of the install date, and a system installed by 31 December 2030 still gets those 12 months. The STC deeming period covers the same slope for panels.

Eligibility, including replacements

• The model is on the register, and the install date falls inside its eligible-from and eligible-to dates.
• Capacity. A heat pump of no more than 425 L. A solar water heater of up to 700 L, or over 700 L with two statutory declarations: one from the owner on ownership and installation, and one on the model, its capacity and the installation.
• Installed exactly as listed, by an installer who meets the state or territory's licensing requirements. For hot water that means the plumbing and electrical licences the state requires.
• Replacing an electric, gas or solar system. The count does not depend on what was there before. The register figure for the model is worked out against an equivalent electric water heater in every case, and neither the Act (compilation of 1 January 2026) nor the regulations (compilation of 1 October 2026) make the number depend on what the new system replaces: it is set by model, zone and install year alone. The REC Registry records the category of the install: part of a new building, replacing an electric hot water system, replacing a gas hot water system, replacing a solar water heater including a boosted one, the first water heater at an existing building, or other. State rebate schemes have replacement rules of their own; the STC count does not.
• Once only. A solar water heater is taken to be installed once in its life, so a second-hand or relocated unit earns nothing.
• Within 12 months. Certificates must be created within 12 months of the install date, and the regulator suggests using a registered agent if you are within 8 weeks of it.

The regulator also notes that state and territory rebates may apply on top of STCs, and points to the Smart Energy Council's list of them. Our state pages cover the programs we know of beside STCs, mostly for panels and batteries.

The paperwork

A hot water claim needs less than a solar panel claim, and the regulator still fails claims that are incomplete, incorrect or ineligible, and can ask for the documents for up to 5 years. Its guideline on collecting mandatory information for solar water heaters (version 1.1, 25 January 2024) lists what the REC Registry asks for, and its page on creating STCs (2 November 2026) lists the documents.

• The STC assignment form. Under the Act, the owner of the system when it is installed is entitled to create the certificates and may assign that right to another person by written notice. The form is that notice: it names the registered agent, is completed after the install is complete, and must state the financial incentive or the regulator may fail the certificates. The regulator's sample forms with written statements from designers, installers and retailers are for solar PV and batteries. For hot water, agents build their own form from the regulator's generic assignment form (CER-RET-016, version 1.1, 24 May 2017), which carries a ten-year option that applies only to solar water heaters and air source heat pumps, and from the guideline. The STC assignment form explains the declaration and the signatures.
• What the registry asks for. The owner's name, address and contact details. The install address, in enough detail for a regulator's officer to visit. The property type: residential, school or commercial. Single or multi storey. Whether there is more than one solar water heater at the address. The install date. The category of the install. The brand, model, serial numbers, tank size and number of collectors. Whether the capacity exceeds 700 L. The owner's confirmation that the right is assigned for ten years. The number of STCs. The installer's own name, business, address and contact details.
• A certificate of compliance, or its equivalent, from the installer. The regulator's words. In practice that is the plumbing compliance certificate where the state issues one, and the electrical safety certificate for the wiring wherever electrical work was done. Each state names its electrical certificate differently: our state pages list them.
• An invoice from the installer, or a date-stamped photo of the unit. The regulator's photo rule applies: time and date metadata, and geolocation at the install site.
• Over 700 L. The two statutory declarations above.
• Fees and records. The REC Registry charges 8 cents a certificate for solar water heaters, against 47 cents for small generation units, once an account passes its first 250 certificates, and then on all of them. Keep everything for at least 5 years.

Selling hot water STCs

The routes are the three in how to sell STCs. Assign the right to a trader or agent, who pays a price per certificate once the claim is complete. Create and sell the certificates yourself with a REC Registry account, and carry the fees, the audit and the wait. Or list registered certificates in the clearing house at $40 before GST, with no guarantee of when they sell. For a plumber or a hot water retailer the first route is the usual one: the owner signs the assignment form on the day, the discount comes off the invoice, and the trader pays per certificate.

The regulator does not set the price agents pay and does not step into disputes between owners and agents, so the contract is what protects you. What to check is the same as for any trader: the price and whether it includes GST, when it pays, what comes off, who carries a failed claim, and that the agent named on the form is the business you deal with. How to choose an STC trader.

We buy hot water STCs. $41 per STC incl GST on a home system, the same as a panel or battery STC, paid within 1 business day of an approved claim, with nothing taken off and no lock-in or minimum. That is above the $40 clearing house cap, and the regulator advises caution with prices above it, so check us as you would any trader. Hot water has its own register, zones and paperwork, which this guide covers: the assignment form, the installer's certificate of compliance and the invoice or photo come to us with the job. Open a trader account, or tell us what you install and we will set you up.