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Solar Greenhouse Calculator

Calculate the PV system size and annual energy to run a heated UK greenhouse. Free calculator with MCS-grade U-values, climate degree-days, and payback.

Solar Greenhouse Calculator

Heating load (thermal)
47,040 kWh
Heating electricity
16,800 kWh
Lighting electricity
8,400 kWh
Fans + circulation
500 kWh
Total electricity / year
25,700 kWh
Annual electricity bill
£6,939
Recommended PV size
32.2 kW
PV installed cost
£54,807
Simple payback (years)
7.9

How this calculator works

A heated UK greenhouse has three electric loads: space heating (when run from a heat pump rather than gas), supplemental lighting, and ventilation. Enter ten values and the calculator returns the annual kWh of each load, the PV system size required, the installed PV cost, and a simple payback in years.

  1. Greenhouse floor area in m² — the heated footprint only.
  2. Glazing material — single polythene, double-polythene inflated, twin-wall polycarbonate, or double-glazed glass. Each uses a measured U-value from the CIBSE Guide A 2021 and Energy Saving Trust greenhouse retrofit data.
  3. Climate zone — cold (Inverness, Aberdeen), moderate (Birmingham, Manchester), or mild (Brighton, Cornwall). The dropdown loads the relevant heating degree days base 18°C.
  4. Heating COP — 1.0 for resistance, 2.8 for an air-source heat pump at UK winter conditions (per RHPP-2 monitored data), or 0 to skip heating from the PV calculation if you use LPG, oil, or natural gas.
  5. Supplemental lighting W/m² — 60–80 W/m² for leafy greens, 150–250 W/m² for tomatoes, peppers, and herbs.
  6. Lighting hours per day — supplemental hours added to natural daylight, usually 4–8 hours through October to March.
  7. Lighting active days per year — typically 180–220 days through the heating season.
  8. Electricity rate in £/kWh — Ofgem price cap residential rate, or whatever your fixed tariff is.
  9. Peak sun hours per day — 2.5 in northern Scotland, 2.8 in the Midlands, 3.0 in southern England (Met Office solar irradiance climatology).
  10. PV installed cost per kWp — MCS-installed turnkey price. Solar Energy UK’s 2026 commercial PV cost report quotes £1,500–£1,800/kWp for 4–20 kWp residential.

The maths is first-principles: thermal heating load = U × envelope_area × HDD × 24 / 1000. We assume the greenhouse envelope (sidewalls + roof + endwalls) is twice the floor area. Lighting kWh = (W/m² × m²) / 1000 × hours × days. Fan and circulation load is fixed at 5 kWh/m²/year per Energy Saving Trust monitored greenhouse audits.

Why solar is suddenly a UK greenhouse story

Until 2022, UK commercial greenhouses ran almost exclusively on natural gas or red diesel. Three things changed.

First, gas prices roughly doubled after the 2022 wholesale shock and remain 60 percent above 2019 levels. The Ofgem price cap on standard tariff electricity sits at around £0.27/kWh, but a heat pump at SCOP 2.8 delivers heat at an effective £0.096/kWh — back below pre-2022 gas economics.

Second, MCS-certified PV installation costs have dropped to roughly £1,500–£1,800/kWp turnkey for 4–20 kWp systems. The Smart Export Guarantee (SEG) replaced the legacy Feed-in Tariff with export rates of £0.05–£0.15/kWh depending on supplier — Octopus Outgoing Fixed pays the high end. That makes PV economically meaningful even without battery storage.

Third, the Boiler Upgrade Scheme grants £7,500 toward an MCS-certified air-source heat pump. ECO4 covers loft and wall insulation for eligible households. The greenhouse heat pump is a domestic-applicable installation if the greenhouse is attached to a dwelling.

The combined effect is that a fully-electrified PV + heat pump greenhouse now pencils at a 7–9 year payback in the Midlands and 10–12 years in northern Scotland. That is well inside the 20–25 year service life of polycarbonate glazing and the 25-year MCS warranty on PV modules.

Glazing U-values — the operating cost driver

The glazing decision drives 60–75 percent of greenhouse operating cost. CIBSE Guide A 2021 and the Solar Energy UK 2024 commercial greenhouse guide list:

  • Single polythene film — U = 6.0 W/m²·K, 3–4 year life, £5–£8/m². Cheapest install, highest running cost.
  • Double polythene inflated — U = 4.0 W/m²·K, 4–6 year life, £10–£15/m². Standard for commercial growers.
  • Twin-wall polycarbonate — U = 3.5 W/m²·K, 12–15 year life, £25–£40/m². Best long-term value for domestic and small commercial.
  • Double-glazed glass — U = 2.8 W/m²·K, 25+ year life, £120–£200/m². Only justified for high-value crops in cold zones.

A 100 m² polycarbonate greenhouse with a 2× envelope factor has 200 m² of skin. In a Midlands climate (HDD18 ≈ 2500 K·day) annual heating is:

  • Single polythene: 6.0 × 200 × 2500 × 24 / 1000 = 72,000 kWh thermal/yr
  • Twin-wall polycarbonate: 3.5 × 200 × 2500 × 24 / 1000 = 42,000 kWh thermal/yr

Switching saves 30,000 kWh thermal annually. At SCOP 2.8 that is 10,700 kWh electric, worth £2,890 per year at £0.27/kWh. The polycarbonate upgrade pays for itself within 5 years.

Supplemental lighting — the UK lattitude problem

UK greenhouses sit at 50–58°N latitude, where mid-December delivers only 7 hours of daylight at low solar elevation. That means winter supplemental lighting is unavoidable for anything beyond winter-hardy leafy greens. CIBSE TM52 and Defra horticultural lighting guidance suggest:

  • Leafy greens (lettuce, kale, spinach, rocket): 60–80 W/m² LED, 6 hours/day supplemental October–March.
  • Herbs (basil, coriander, mint, parsley): 80–100 W/m², 6 hours/day.
  • Tomatoes, peppers, aubergines: 150–250 W/m², 8–10 hours/day — economically marginal for hobby greenhouses.
  • Microgreens and seedling propagation: 100–150 W/m², 12 hours/day on staged benches.

Modern LED fixtures (Heliospectra Eos, Valoya BX-series, Fluence SPYDR) deliver 2.8–3.0 µmol/J. HPS at 1.7 µmol/J should be retrofitted to LED before sizing PV — the energy saving alone repays the retrofit in under 2 years.

Worked example — 100 m² polycarbonate greenhouse in Birmingham

A Midlands homesteader’s 100 m² polycarbonate greenhouse, heat-pump heated, growing leafy greens through the winter:

  • Glazing: twin-wall polycarbonate, U = 3.5 W/m²·K
  • Envelope: 200 m²
  • Climate: moderate (Midlands), HDD18 = 2500 K·day
  • Heat pump: SCOP 2.8 (Ecodan or similar)
  • Lighting: 70 W/m² LED, 6 hr/day, 200 days/yr
  • Fans + circulation: 5 kWh/m²/yr

Computations:

  • Heat thermal: 3.5 × 200 × 2500 × 24 / 1000 = 42,000 kWh/yr
  • Heat electric: 42,000 / 2.8 = 15,000 kWh/yr
  • Lighting: (70 × 100 / 1000) × 6 × 200 = 8,400 kWh/yr
  • Fans: 100 × 5 = 500 kWh/yr
  • Total electric: 23,900 kWh/yr
  • Annual bill at £0.27/kWh: £6,453
  • PV size: 23,900 / (2.8 × 365 × 0.78) = 30 kWp
  • PV cost at £1,700/kWp: £51,000
  • Simple payback: 51,000 / 6,453 = 7.9 years

The 30 kWp size is large for a domestic site — roof or ground-mounted area of ~150 m² of panels. Most domestic UK roofs cap out at 10–15 kWp, so plan a hybrid grid + PV solution rather than full PV offset.

Common UK sizing mistakes

  • Using SAP weather data for greenhouses. SAP assumes 19°C indoor set-point. Greenhouse setpoints are often 10–14°C, so HDD-adjusted to setpoint is much lower. Use the calculator’s setpoint adjustment or look up CIBSE TM41 horticultural HDD tables.
  • Quoting PV at residential 4 kWp. Greenhouse loads are 3–10× a typical dwelling. You will need a commercial install permit if PV exceeds 16 A single-phase G98, or G99 application for anything over 11 kWp three-phase.
  • Ignoring DNO connection costs. Western Power Distribution, UKPN, and SP Energy Networks all charge £600–£3,000 for G99 applications. Budget for it.
  • Skipping the SEG arbitrage. Octopus Outgoing Fixed pays £0.15/kWh export for SEG-registered installations, which dramatically improves PV payback over the standard £0.05/kWh.
  • Pairing PV with oil or LPG heating. PV won’t offset the fuel cost. Set COP = 0 in the calculator and run a separate fuel analysis. Switching to ASHP is almost always the right move before adding PV.

Stacking UK incentives in 2026

  • Boiler Upgrade Scheme — £7,500 toward an MCS-certified air-source heat pump.
  • Smart Export Guarantee — £0.05–£0.15/kWh export rate on surplus PV generation. Octopus Outgoing Fixed currently leads.
  • 0% VAT on energy-saving materials — applies to PV, batteries, heat pumps installed at residential premises through March 2027.
  • ECO4 — up to £14,000 toward insulation and heating measures for eligible households.
  • Sustainable Farming Incentive (Defra) — covers PV and battery storage on farm greenhouses through the SFI Capital Grants stream.

Effective net cost on a £42,000 PV + battery + ASHP greenhouse package after stacking can drop below £25,000. That is the math driving the 3× increase in MCS-certified greenhouse PV installations across England and Wales between 2022 and 2025.

Sources

Frequently asked questions

How many solar panels do I need to run a UK greenhouse?
A typical 100 m² polycarbonate hobby greenhouse heated to 12°C with an air-source heat pump (SCOP 2.8) plus 70 W/m² of LED supplemental light for 6 hours over 200 winter days needs about 19,000 kWh of electricity per year. With UK peak sun hours of 2.8 a 21 kW PV array is required — roughly 52 panels at 400 W each — and at MCS-installed cost of £1,700/kWp that is £35,700 capex. With a 13.5 kWh GivEnergy or Tesla battery the payback at £0.27/kWh runs at 7–8 years, before any Smart Export Guarantee credit.
Is a heat pump worth it for a UK greenhouse?
Yes when the alternative is electric resistance or LPG heating. A Mitsubishi Ecodan or Daikin Altherma cold-climate ASHP delivers a Seasonal Coefficient of Performance (SCOP) of 2.5 to 3.2 at UK winter conditions, monitored across the Energy Saving Trust's RHPP-2 field trial. That cuts greenhouse heating bills by 60–70 percent versus resistance. The Boiler Upgrade Scheme grants £7,500 toward an MCS-certified ASHP install. The pump pays back inside 5–7 years on a greenhouse application versus paying for resistance heating at the standard variable tariff of £0.27/kWh.
Which greenhouse glazing has the lowest U-value?
Twin-wall polycarbonate sheets deliver U = 3.5 W/m²·K at roughly £25–35/m² supplied — the best balance of insulation and cost. Standard horticultural glass is 6.0 W/m²·K. Double-glazed glass drops to 2.8 but costs three times as much. Polythene film at U = 6.0 is the cheapest at install but needs replacing every 3–4 years and pushes heating cost roughly 70 percent higher than polycarbonate. The Energy Saving Trust's 2024 commercial greenhouse retrofit guide rates twin-wall polycarbonate as the most economic glazing for greenhouses heated below 12°C in CIBSE Climate Zone B2.
Can I grow tomatoes through a UK winter with solar lighting?
Only with substantial supplemental light and heat. UK December sees about 1 hour of useful PAR daylight at 52°N latitude — far below the 12–14 hour photoperiod tomatoes need. You can do it with 200 W/m² of LED grow lighting on a 12-hour photoperiod, but the load is huge: a 50 m² tomato bench needs 10 kW of LED running 12 hours, or 36,500 kWh/year. That alone requires 40 kWp of PV at UK irradiance, which is unrealistic on a domestic site. Leafy greens, herbs, and brassicas grow happily through the winter on 60–80 W/m² supplemental for 4–6 hours, which is sized-able with 10–15 kWp of PV.
How much does a solar-powered UK greenhouse cost in 2026?
A turnkey 100 m² polycarbonate greenhouse with 10 kWp MCS-installed PV, a 13.5 kWh battery, and a Mitsubishi Ecodan 8.5 kW cold-climate ASHP runs about £42,000–£58,000 including 20% VAT. The greenhouse structure is £12,000–£18,000, PV at £1,700/kWp installed is £17,000, battery £8,500, and ASHP £11,000–£16,000. The Boiler Upgrade Scheme covers £7,500 of the ASHP, ECO4 may cover up to £5,000 of insulation, and SEG export pays £0.05–£0.15/kWh on surplus generation.

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