Solar Thermal vs Solar PV for Hot Water (Australia)

Compare solar thermal hot water vs solar PV + heat-pump for Australian homes. Free calculator with CEC-listed costs, STC rebate and payback by climate zone.

Solar Thermal vs PV — Hot Water Comparator

Annual hot water demand (kWh) Backup fuel price (per kWh) Backup fuel efficiency (%) Electricity rate / feed-in tariff (per kWh) Annual energy price rise (%) System lifetime (years)

Solar thermal (CEC-listed evacuated tube / flat plate)

Thermal system cost Annual solar fraction (%) STC rebate (%)

Net cost$3,360

Year-1 savings$110

PaybackBeyond lifetime

Lifetime savings$2,877

Solar PV + heat-pump hot water

PV system cost PV annual production (kWh) Heat-pump COP STC rebate (%)

Net cost$2,100

Year-1 savings$522

Payback3.9 years

Lifetime savings$13,989

Verdict

Solar PV + heat pump wins on lifetime value

How to use this calculator

Enter your household hot water energy demand and the backup fuel you’d otherwise burn (natural gas, LPG, or electric resistance). The calculator runs two parallel ROI tracks and tells you which pathway saves more money over the system lifetime under Australian conditions — CEC-listed solar thermal collectors, or solar PV plus a heat-pump hot-water unit.

What each input means:

Annual hot water demand (kWh) — useful heat delivered to taps. A typical 4-person Australian household uses about 3,500 kWh per year (Sustainability Victoria field data). Multiply household occupants × 875 kWh if you don’t know yours.
Backup fuel price (per kWh) — Australian natural gas residential averaged 3.8c per MJ in early 2026 (AER 2025 State of the Energy Market) ≈ A$0.038 per kWh of fuel. LPG runs A$0.10-A$0.14 per kWh. Electric resistance at your retail rate (33c per kWh nationally).
Backup fuel efficiency — 85% for an instantaneous gas water heater (AGA 5-star minimum 96% for new); 100% for electric resistance; 300%+ for a heat pump.
Electricity rate / feed-in tariff — for valuing PV exports. Use your live feed-in tariff (3-7c/kWh under most retailers in 2026; some retailers offer time-of-export rates up to 12c).
Annual energy price rise — Australian residential electricity rose 3.5% annually 2020-2025 per AER data. Gas a similar trajectory. Use 3.5% as a reasonable default.
System lifetime — CEC product approval and STC eligibility require minimum 10-year warranty on solar water heaters; typical real-world life is 15-20 years with one anode/element swap. PV panels carry 25-year output warranty.

How the math works

Both pathways are scored against the same baseline — cost per kWh of useful heat delivered by your backup fuel:

effective_rate_per_kWh_useful = fuel_rate / efficiency

For Australian gas at A$0.038/kWh in an 85%-efficient unit: 0.038 / 0.85 = A$0.045 per kWh of delivered heat.

Solar thermal pathway:

annual_useful_heat_saved = solar_fraction × hot_water_demand
annual_cost_saved        = annual_useful_heat_saved × effective_rate
net_cost                 = system_cost × (1 - STC_discount%/100)
year_n_savings           = annual × (1 - 0.007)^(n-1) × (1 + escalation)^(n-1)
lifetime_savings         = Σ year_n_savings for n = 1..lifetime

Australian evacuated-tube collectors degrade approximately 0.7% per year, mostly from accumulated dust and seal aging.

PV + heat-pump pathway:

DHW_served_by_PV  = min(PV_production × COP, hot_water_demand)
PV_used_for_DHW   = DHW_served_by_PV / COP
PV_excess         = PV_production - PV_used_for_DHW
annual_cost_saved = DHW_served × effective_rate + PV_excess × FiT
year_n_savings    = annual × (1 - 0.005)^(n-1) × (1 + escalation)^(n-1)

Australian PV degrades at 0.5% per year — Clean Energy Council Approved Solar Retailer benchmark.

The calculator picks a winner by comparing lifetime net gain on each pathway. Within 5% it calls it a tie.

Worked Australian example (Brisbane, mains gas backup)

Inputs:

4-person household, hot water demand 3,500 kWh/year of useful heat
Mains gas A$0.038/kWh, 85% efficiency → A$0.045 per kWh useful
Solar thermal: 30-tube evacuated + 300 L tank, A$5,500 before STCs, A$4,400 after, 75% solar fraction in Brisbane
Solar PV: 1.5 kWp, 2,400 kWh/year, A$3,000 after STCs, COP 3.2 HPWH
3.5% escalation, 20-year lifetime

Solar thermal:

Year-1 savings: 0.75 × 3,500 × A$0.045 = A$118
Net cost after STCs: A$4,400
Lifetime savings (with degradation + escalation): ~A$3,200
Payback: never within 20 years
Net loss: ~A$1,200

Solar PV + HPWH:

DHW served: min(2,400 × 3.2, 3,500) = 3,500 kWh (PV fully covers demand)
PV used for DHW: 3,500 / 3.2 = 1,094 kWh; excess = 1,306 kWh
Year-1 savings: 3,500 × A$0.045 + 1,306 × A$0.05 (FiT) = A$223
Net cost after STCs: A$3,000
Lifetime savings: ~A$6,100
Payback: ~12 years
Net gain: ~A$3,100

PV+HPWH wins by roughly A$4,300 over 20 years in this Brisbane example. The pattern holds across most Australian capitals when gas is cheap.

When solar thermal still wins in Australia

Solar thermal beats PV+HPWH in three Australian situations:

Far-north Queensland and Top End (Darwin, Cairns, Townsville) — solar fraction above 80% means thermal essentially covers all hot water year-round. With gas often unavailable and LPG expensive, thermal is the simplest install.
Off-grid or DNSP export-limited installations — some Queensland and SA networks now zero-export limit residential PV (DEW 2024 guidelines, Energex 5 kW limit policy). When PV exports earn nothing, thermal regains the advantage because it self-consumes via tank temperature.
LPG- or electric-resistance backup at retail rate. LPG at A$0.13/kWh of fuel input makes the “backup fuel” so expensive that even a low-fraction thermal install pays back in 7-9 years.

When PV + heat pump wins (most Australian homes)

PV+HPWH is the better choice when:

Mains gas is your current backup
You have a worthwhile feed-in tariff (>5c/kWh)
Your roof has room for a full residential PV system (6.6-10 kWp is typical)
You want flexibility for EV charging or future electrification of cooking/space heating

The big swing factor is Australia’s gas phase-out. Victoria has banned new gas connections in new homes from 2024; ACT followed in 2025. Across the country, gas prices are forecast to rise faster than electricity through 2030. PV is a future-proof asset; solar thermal is locked to one job that’s increasingly competing with cheaper electricity.

Australian regional reference (4-person household, 3,500 kWh demand)

Region	Climate	Best pathway	Why
Darwin / Top End	Tropical	Solar thermal	80%+ solar fraction; no gas grid; LPG expensive
Cairns / Townsville	Tropical wet	Solar thermal or PV+HPWH	Both excellent; thermal slightly simpler
Brisbane / SE Queensland	Subtropical	PV + HPWH	Gas cheap; PV yields 1,500+ kWh/kWp
Sydney / Newcastle	Temperate	PV + HPWH	NSW retail rates favour PV self-consumption
Melbourne / Geelong	Cool temperate	PV + HPWH	Lower thermal fraction; Victorian gas phase-out makes PV future-proof
Adelaide	Mediterranean	PV + HPWH	Good thermal too but DNSP export limits favour PV+HPWH self-consumption
Perth / SW	Mediterranean	Either pathway works	High thermal fraction; PV yields too
Hobart / Tasmania	Cool temperate	PV + HPWH (with hydro backup)	Thermal fraction drops to 55%; TAS has cleanest electricity
Regional NT / WA / outback	Hot, off-grid common	Solar thermal	Cheap install, no STC dependency on grid connection

Hybrid PVT collectors in Australia

Australian PVT (photovoltaic-thermal) is a small but growing segment. CEC-approved manufacturers include Sunmaxx and Naked Energy through Australian distributors. Per-panel installed cost runs A$1,500-A$2,200. PVT is the answer when roof area is the binding constraint, particularly on Sydney terraces or strata-titled inner-city homes.

For most Australian homeowners with mains gas, solar PV + heat-pump hot water is the higher-ROI choice under 2026 conditions. Run the calculator with your actual gas rate, FiT, and STC-net quotes before signing.

Solar pool heating calculator — same physics for pools
Solar panel payback calculator — full-house PV payback under Australian FiTs
Cost of solar panels calculator — A$/kW benchmarks after STCs

Sources

Clean Energy Council Approved Solar Retailers — installer quality framework and product approval
Clean Energy Regulator — Small-scale Renewable Energy Scheme — STC mechanics for both pathways
Australian Energy Regulator — State of the Energy Market 2025 — residential gas and electricity prices
Sustainability Victoria — Hot water systems — household demand benchmarks
SunWiz Solar Insiders Report 2025 — A$/kW PV installed costs by state

Frequently asked questions

Is solar thermal or solar PV better for hot water in Australia?

For most Australian homes installed after 2022, solar PV paired with a heat-pump hot water unit beats stand-alone solar thermal on 20-year ROI. A 1.5 kWp PV array installed under CEC rules costs around A$3,000 after STC discounts, generates ~2,200 kWh per year in the average Australian climate, and feeds an HPWH at COP 3.2 — delivering 7,000+ kWh of useful heat against a household demand of around 3,500 kWh. A CEC-listed solar thermal system (close-coupled evacuated-tube or flat-plate + 300 L tank) costs A$4,000-A$5,500 fitted after STCs and covers 65-75% of annual demand. PV also exports excess generation under feed-in tariffs; thermal cannot.

How do Small-scale Technology Certificates (STCs) apply to each pathway?

STCs apply to both. Solar PV STCs are based on system size × deeming period × zone rating (typically 4-6 kW residential gets 70-120 STCs worth A$2,500-A$4,500 at A$36-A$40 spot). Solar water heater STCs are awarded under the Small-scale Renewable Energy Scheme using TRNSYS-modelled performance values per CER guidelines — typically 20-35 STCs worth A$700-A$1,300. The STC scheme legislatively winds down to zero by 31 December 2030, so both pathways lose this support over time.

What size PV array do I need to fully replace gas hot water in Australia?

For a typical 4-person Australian household using 3,500 kWh of useful heat per year, you need around 1,100-1,400 kWh of PV electricity to feed an HPWH at COP 3.2 — that's 0.8-1.0 kWp of panels in Brisbane or Sydney, or 1.0-1.2 kWp in Melbourne or Hobart. Most homeowners size the total PV system to 6.6 kWp or 10 kWp anyway (the common inverter limit thresholds under DNSP rules), so dedicated DHW sizing rarely matters — the HPWH becomes one of many loads pulling from a generously-sized array.

What's the realistic solar fraction for solar thermal in Australia?

Australia is one of the few places where solar thermal still delivers respectable lifetime returns because of the climate. CEC-listed evacuated-tube collectors typically deliver: Darwin and Cairns 80-85%, Brisbane and Perth 70-75%, Sydney and Adelaide 65-70%, Melbourne 55-65%, Hobart 50-60%. The calculator above defaults to 70% which fits the Australian average. The annual average is genuinely high enough that thermal can compete on lifetime ROI — what tips the math is whether gas is your backup or electric resistance.

What does NSW Energy Savings Scheme add for either pathway?

NSW residents replacing an electric storage water heater can claim ESS certificates worth A$800-A$1,500 toward a heat-pump unit or a solar thermal system. Victoria has a similar Solar Homes program offering rebates of A$1,000 for solar water heaters and A$1,400 for heat pumps. SA and Queensland have smaller, capped schemes. These stack on top of STCs and can shift the math significantly for thermal vs PV+HPWH — always check the live rebate state for your postcode at energy.gov.au.