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Solar Greenhouse Calculator

Calculate the PV system size and annual energy to run a heated Australian greenhouse. Free calculator with CEC-grade U-values and STC-adjusted payback.

Solar Greenhouse Calculator

Heating load (thermal)
25,200 kWh
Heating electricity
9,000 kWh
Lighting electricity
2,250 kWh
Fans + circulation
500 kWh
Total electricity / year
11,750 kWh
Annual electricity bill
$3,525
Recommended PV size
9.2 kW
PV installed cost
$11,923
Simple payback (years)
3.4

How this calculator works

A heated Australian greenhouse has three electric loads: space heating (when needed in cool regions), supplemental lighting (mainly for winter leafy greens), and ventilation (much larger than in cool climates). Enter ten values and the calculator returns annual kWh of each load, PV size required, installed cost, and a simple payback in years.

  1. Greenhouse floor area in m² — the conditioned footprint.
  2. Glazing material — single polyethylene film, double-poly inflated, twin-wall polycarbonate (most common in Australia), or double-glazed glass.
  3. Climate zone — cold (Hobart, Canberra), moderate (Melbourne, Adelaide), mild (Sydney, Perth), or tropical (Cairns, Darwin).
  4. Heating COP — 1.0 for resistance, 2.8 for an air-source heat pump (typical R32 split at Australian winter ambient), or 0 to skip heating from the PV calc if you use LPG or no supplemental heat.
  5. Supplemental lighting W/m² — 40–60 W/m² for leafy greens in southern Australia, 0 for tropical/sub-tropical sites.
  6. Lighting hours per day — 2–4 hours supplemental in winter, 0 from October to March.
  7. Lighting active days per year — typically 120–180 days through the southern winter.
  8. Electricity rate in A$/kWh — Origin, AGL, Red Energy, or your retailer’s variable tariff.
  9. Peak sun hours per day — 4.0 in Hobart, 4.5 in Melbourne and Sydney, 5.0 in Brisbane and Perth, 5.5 in Adelaide, 6.0 in Alice Springs and Darwin.
  10. PV installed cost per kW — Clean Energy Council median for 6–13 kW residential PV after STC rebate runs A$1,200–A$1,400/kW.

The maths is first-principles: thermal heating = U × envelope_area × HDD × 24 / 1000. We assume the greenhouse envelope (sides + roof + endwalls) is 2× the floor area. Lighting kWh = (W/m² × m²) / 1000 × hours × days. Fan and circulation load is fixed at 5 kWh/m²/year per Clean Energy Council monitored data on Victorian commercial greenhouses.

Why solar pencils for Australian greenhouses

Australia has the highest residential solar penetration in the world — over 35 percent of detached dwellings carry rooftop PV. Three factors make greenhouse PV particularly attractive.

First, peak sun hours of 4.5 to 6.0 across the populated southeast and southwest are roughly double the UK and 30 percent higher than the US Mid-Atlantic. A 1 kW panel produces about 1,500–1,800 kWh/year in Sydney versus 950 kWh/year in Berlin.

Second, the Small-scale Technology Certificates (STC) scheme under the Renewable Energy Target reduces the upfront PV cost by 30–35 percent. STC value is roughly A$36–A$40 each in 2026, and a 6 kW system in Zone 3 (Sydney, Brisbane) accrues about 80 STCs — worth A$3,000–A$3,200 off the install.

Third, Australian retail electricity prices have risen from A$0.22/kWh in 2019 to A$0.28–A$0.34/kWh in 2026 (regional averages from Australian Energy Regulator), while feed-in tariffs have collapsed from A$0.55 to A$0.05–A$0.07/kWh. The economic case shifted from exporting to self-consuming, and a heated greenhouse is one of the few residential loads that can soak up midday PV generation.

The combined effect is that a PV-paired greenhouse in southern Australia now pencils at a 5–7 year payback, well inside the 25-year STC-eligible panel warranty.

Glazing choice — UV-rated for Australia

Australian UV is roughly 30 percent higher than the UK on an annual integrated basis. That kills standard horticultural polyethylene within 3 years unless it carries an EVA + UV-stabiliser top coat. Practical choices from the Australian Hydroponic and Greenhouse Association 2024 guide:

  • Single polyethylene (UV-stabilised) — U = 6.0 W/m²·K, 3-year life, A$8–A$12/m². Acceptable for ephemeral commercial crops; not for hobby.
  • Double polyethylene inflated — U = 4.0 W/m²·K, 5-year life, A$18–A$25/m². Standard for large commercial greenhouses.
  • Twin-wall polycarbonate — U = 3.5 W/m²·K, 12–15 year life, A$45–A$70/m². The default for hobby and high-value commercial.
  • Double-glazed glass — U = 2.8 W/m²·K, 25+ year life, A$180–A$260/m². Mostly used in research and very high-value crops; AS/NZS 1170 hail design becomes critical.

A 100 m² polycarbonate greenhouse with 2× envelope = 200 m². In Melbourne (HDD18 ≈ 1500 K·day):

  • Single poly: 6.0 × 200 × 1500 × 24 / 1000 = 43,200 kWh thermal/yr
  • Twin-wall polycarbonate: 3.5 × 200 × 1500 × 24 / 1000 = 25,200 kWh thermal/yr

Switching saves 18,000 kWh thermal annually. At COP 2.8 that is 6,430 kWh electric, worth A$1,930/yr at A$0.30/kWh — recouping the polycarbonate upgrade in 4 years.

Supplemental lighting — sized to Australian latitude

Australian greenhouses range from 16°S (Cairns) to 43°S (Hobart). Winter daylight at 43°S is 9 hours minimum — enough for most leafy crops without supplemental light. Practical guidelines from the Australian Centre for Plant Functional Genomics:

  • Leafy greens (lettuce, rocket, baby spinach): 40–60 W/m² LED, 2–4 hours/day supplemental June–August in TAS/VIC, none in QLD/NT.
  • Brassicas (broccoli, kale): 50–80 W/m², 3 hours/day in southern winter.
  • Tomatoes, capsicums: 100–150 W/m², 4–6 hours/day if winter cropping in cool zones — economically marginal.
  • Strawberries: 60–80 W/m², 4 hours/day for late autumn extension.

LED efficacy in 2026 — Heliospectra, Valoya, and locally manufactured Bioledex — sits at 2.6–3.0 µmol/J. Retrofit HPS to LED before sizing PV.

Worked example — 100 m² polycarbonate greenhouse in Melbourne

A southern Victorian homesteader’s 100 m² polycarbonate greenhouse with leafy greens and herbs through winter:

  • Glazing: twin-wall polycarbonate, U = 3.5 W/m²·K
  • Envelope: 200 m²
  • Climate: moderate (Melbourne), HDD18 = 1500 K·day
  • Heat pump: COP 2.8 (R32 split system at Australian winter)
  • Lighting: 50 W/m² LED, 3 hr/day, 150 days/yr
  • Fans + circulation: 5 kWh/m²/yr

Computations:

  • Heat thermal: 3.5 × 200 × 1500 × 24 / 1000 = 25,200 kWh/yr
  • Heat electric: 25,200 / 2.8 = 9,000 kWh/yr
  • Lighting: (50 × 100 / 1000) × 3 × 150 = 2,250 kWh/yr
  • Fans: 100 × 5 = 500 kWh/yr
  • Total electric: 11,750 kWh/yr
  • Annual bill at A$0.30/kWh: A$3,525
  • PV size: 11,750 / (4.5 × 365 × 0.78) = 9.2 kW
  • PV cost at A$1,300/kW (after STCs): A$11,960
  • Simple payback: 11,960 / 3,525 = 3.4 years

That payback is exceptional — among the best in the world. Australian PV is cheap, electricity is expensive, and STCs front-load the savings.

Common Australian sizing mistakes

  • Designing for southern HDDs in Queensland. Brisbane and northwards have HDD18 below 500 K·day; the calculator’s “tropical” zone is correct. Don’t over-heat.
  • Forgetting ventilation loads. Australian summers regularly exceed 35°C. Exhaust fans plus evaporative cooling can consume 15–25 kWh/m²/year — 3–5× the fan load assumed by default. For Queensland and tropical sites, increase fan baseline to 20 kWh/m²/year.
  • Skipping the STC rebate in the cost field. The CEC-installed price already includes the STC discount. Don’t double-count.
  • Ignoring DNSP fees. Embedded generation under 5 kW falls under inverter standards AS/NZS 4777.2 with no fee. Above 5 kW the DNSP (SAPN, Essential Energy, Endeavour Energy, Energex, Western Power, Tasnetworks) requires a formal connection enquiry — typically A$200–A$2,000 depending on the network.
  • Hail risk on glass. AS/NZS 1170.2 ULS hail design is critical for glass greenhouses in NSW, VIC, QLD, and WA. Polycarbonate is the safer choice for hail-prone regions.

Stacking Australian incentives in 2026

  • Small-scale Technology Certificates (STCs) — 30–35% off PV install upfront. Worth A$3,000–A$4,000 on a 6–8 kW system in Zone 3.
  • Federal Battery Booster (proposed 2026) — A$2,400 per household for new battery installs of 5 kWh+, subject to passage.
  • NSW Empowering Homes — interest-free loan up to A$14,000 for solar + battery in NSW.
  • VIC Solar Homes — A$1,400 rebate plus A$1,400 interest-free loan for solar PV; A$2,950 rebate for battery storage.
  • QLD Battery Booster — A$3,000 rebate for battery storage (income-tested).
  • WA Distributed Energy Buyback Scheme (DEBS) — feed-in tariff varies by time-of-day, up to A$0.10/kWh peak.

Effective net cost on a A$32,000 PV + battery + heat pump greenhouse package after stacking can drop below A$20,000 in NSW, VIC, and QLD. That is the math driving the 40 percent year-on-year increase in CEC-accredited residential greenhouse PV permits across the eastern states.

Sources

Frequently asked questions

How many solar panels does an Australian greenhouse need?
A typical 100 m² polycarbonate hobby greenhouse in Sydney or Melbourne heated to 10°C with an air-source heat pump (HSPF 9.5) plus 50 W/m² of LED supplemental light for 3 hours over 150 winter days needs about 9,000 kWh of electricity per year. With CEC peak sun hours of 4.5 in Sydney that needs a 7 kW PV array — roughly 17 panels at 415 W each — at CEC-installed cost of A$1,300/kW (after STC rebate) is A$9,100 capex. With CEC-listed STCs and a 13.5 kWh battery the payback at A$0.30/kWh runs at 5–6 years.
Do I need a heat pump for a greenhouse in Australia?
Only in cool-zone regions — Tasmania, Victorian highlands, Adelaide Hills, Canberra, and southern NSW tablelands. The rest of mainland Australia can hold a 10–12°C minimum greenhouse setpoint with passive thermal mass alone (water barrels, concrete floor) plus night curtains. A small 4–6 kW air-to-air heat pump (R32 split system at HSPF 9.5–10.0) is enough to back up cold nights at COP 3.5 average. Brisbane, Perth, and Darwin greenhouses rarely need supplemental heat — ventilation and shade are the bigger loads.
What is the best greenhouse covering for Australian conditions?
Twin-wall polycarbonate at U = 3.5 W/m²·K with a UV-stabilised top coat is the dominant choice for Australian commercial and hobby greenhouses. AS/NZS 4859 lists it at 15-year service life under Australian UV. Horticultural polyethylene film at 200 µm is cheaper but degrades within 3 years under Australian UV without an anti-IR coating. Glass is rare — wind and hail loading under AS/NZS 1170.2 make it impractical except in protected enclosures. For tropical Queensland greenhouses, single 50% shade cloth on top of polycarbonate is the standard combination.
Can I run a greenhouse off-grid in Australia?
Yes for small loads but expect substantial battery cost. A 100 m² greenhouse with 50 W/m² LED, 4 kW backup heat pump, and 0.5 kW circulation fans needs about 12,000 kWh/year. Off-grid sizing at 5 PSH and 3 days autonomy needs roughly 12 kW of PV and 60–80 kWh of LiFePO4 battery (BYD HVM, Sungrow, or BSLBatt cabinets). At CEC retail of A$650/kWh installed that is A$45,000 in batteries alone. Most regional Australian greenhouses go hybrid: 10 kW PV + 10 kWh battery + grid connection at SAPN, Essential Energy, or Endeavour Energy.
How much does a solar-powered Australian greenhouse cost in 2026?
A turnkey 100 m² polycarbonate greenhouse with 7 kW CEC-installed PV (after STCs), a 13.5 kWh BYD or Sungrow battery, and a 5 kW R32 split system runs about A$28,000–A$42,000 in 2026. The greenhouse structure is A$8,000–A$14,000, PV after STC is A$9,100, battery A$10,500 (no national rebate, but NSW Empowering Homes loan covers A$14,000 interest-free), and the split system A$4,500–A$6,500 installed. STC rebate under the Renewable Energy Target reduces PV cost by 30–35 percent.

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