Solar Greenhouse Calculator

How this calculator works

A solar greenhouse needs to cover three electric loads: space heating (when run off a heat pump rather than gas), supplemental lighting, and ventilation. Enter ten values and the calculator outputs the annual kWh of each load, the recommended PV system size, the installed PV cost, and a simple payback in years.

1. Greenhouse floor area in square feet — the heated footprint, not the full lot.
2. Glazing material — single poly film, double-poly inflated, twin-wall polycarbonate, or double-pane glass. Each carries a measured U-value from the ASHRAE 2020 Greenhouse Engineering Handbook.
3. Climate zone — pick cold (Minneapolis), moderate (Pittsburgh, Boulder), mild (Atlanta), or tropical (Miami). The dropdown loads the average annual heating degree days base 18°C used for heat load.
4. Heating COP — 1.0 for resistance baseboard, 2.8 for a typical air-source heat pump at 17°F, or 0 to skip heat from the PV calculation if you burn propane or natural gas. Cold-climate heat pumps in the Mitsubishi Hyper-Heat or Fujitsu XLTH range hold COP 2.0 down to −13°F.
5. Supplemental lighting W/m² — full sun outdoors is about 1000 W/m² PAR-equivalent, and most leafy greens need 100–200 µmol/m²/s, which is roughly 60–100 W/m² of LED grow light. Tomatoes and peppers need 200–400 µmol/m²/s, so 150–250 W/m².
6. Lighting hours per day — supplemental hours added to natural light, typically 3–6 hours in early morning and late afternoon during winter months.
7. Lighting active days per year — usually 150–220 days through the heating season; off in summer.
8. Electricity rate in $/kWh — your retail residential rate, or the wholesale net-metering credit if you have a state-level tariff.
9. Peak sun hours per day — annual average from NREL’s PVWatts. 4.0 in upstate New York, 5.0 in Iowa, 5.5 in Boulder, 6.0 in Phoenix.
10. PV installed cost per kW — turnkey cash price before incentives. EnergySage’s H2 2025 marketplace median, published in 2026, is $2.49/W. Agricultural or business ownership may have different tax treatment; do not apply a homeowner credit to a post-2025 expenditure.

The math is first-principles: heating thermal load is U × envelope_area × HDD × 24 / 1000. We assume the envelope (sides + roof + endwalls) is twice the floor area, which fits typical Quonset, gothic-arch, and gable-end greenhouse geometries within 10 percent. Lighting kWh = (W/m² × m²) / 1000 × hours × days. Fan and circulation load is fixed at 5 kWh/m²/year, based on the University of Connecticut Extension’s monitored greenhouse audits (3 W/m² horizontal airflow fans running 6 hours/day, plus 0.3 air-changes-per-minute exhaust trim).

Why electrify the greenhouse — the 2026 economics

Before 2022, almost every commercial greenhouse in the US Northeast burned LP gas, natural gas, or fuel oil for winter heat — fossil fuel was simply cheaper per BTU than electric resistance. Three things changed.

First, cold-climate heat pumps hit a coefficient of performance of 2.5 to 3.5 even at outdoor temperatures of 5°F. At a COP of 3.0 and grid electricity at $0.13/kWh, the effective cost per delivered BTU is roughly $0.044/therm. Natural gas at $1.40/therm with an 80 percent furnace efficiency is $0.018/therm cheaper — but only if you have a gas connection. For greenhouses on propane (most rural sites), the heat pump beats propane on operating cost in every state except North Dakota and Wyoming.

Second, federal treatment now depends on ownership and the property involved. Section 25D is unavailable for homeowner expenditures after 31 December 2025. A qualifying business or agricultural PV or storage project may instead be analysed under Section 48E, while heat-pump treatment follows separate rules. USDA REAP assistance may also be available to eligible agricultural businesses; confirm the current grant percentage, cap and stacking rules before reducing project cost.

Third, the cost of utility-scale battery storage dropped to under $400/kWh installed for residential applications, which means you can shift midday PV generation to cover evening lighting and pre-dawn heat pump runtime without burning through expensive grid kWh.

The combined effect is that a fully-electrified, PV-powered greenhouse now pencils with a 9 to 12 year simple payback in zone 5 to zone 7, and a 7 to 9 year payback in zone 4. That is competitive with the structure’s own 20 to 25 year service life.

Glazing U-values — the single most important spec

The glazing decision drives 60 to 75 percent of greenhouse operating cost. Below are reference U-values from ASHRAE 90.1 and the New England Greenhouse Conference 2024 proceedings:

• Single polyethylene film — U = 6.0 W/m²·K, 4-year service life, $0.40 to $0.60/sqft. Cheapest to install, most expensive to operate.
• Double polyethylene inflated — U = 4.0 W/m²·K, 4 to 6-year service life, $0.80 to $1.20/sqft. The default for commercial growers when capital is tight.
• Twin-wall polycarbonate — U = 3.5 W/m²·K, 12 to 15-year service life, $4 to $7/sqft. Best long-term economics for hobby and small commercial greenhouses.
• Double-pane glass — U = 2.8 W/m²·K, 25-year service life, $12 to $20/sqft. Only justified for high-value crops like orchids and high-light vegetables in cold zones.

A 1000 sqft greenhouse at 92.9 m² with a 2× envelope factor has 185.8 m² of skin. In a moderate climate (HDD18 = 2800 K·day) the annual heating load is:

• Single poly: 6.0 × 185.8 × 2800 × 24 / 1000 = 74,948 kWh thermal/yr
• Twin-wall polycarbonate: 3.5 × 185.8 × 2800 × 24 / 1000 = 43,712 kWh thermal/yr

Switching from single poly to polycarbonate cuts the thermal load by 42 percent. At a COP-2.8 heat pump and $0.13/kWh electricity that saves roughly $1,450 per year — recouping the glazing upcharge inside 5 years.

Supplemental lighting — sizing without over-buying

The most common mistake in residential and small commercial greenhouses is over-sizing the lighting array. Sunlight already delivers 4–6 hours of high-PAR light in winter even at 45°N latitude. The role of supplemental light is to extend the photoperiod, not to replace daylight. Practical rules of thumb from Cornell CALS and Michigan State Extension:

• Leafy greens (lettuce, kale, spinach): 60–100 W/m² of LED, 4–6 hours/day supplemental.
• Brassicas (broccoli, cauliflower): 80–120 W/m², 4–5 hours/day.
• Tomatoes, peppers, eggplant: 150–250 W/m², 5–8 hours/day.
• Cannabis (where state-licensed): 400–600 W/m², 12–18 hours/day. Cannabis greenhouses need much larger PV arrays — easily 50 W/sqft of installed capacity.

LED efficacy in 2026 is around 2.8–3.0 µmol/J for top-tier full-spectrum fixtures (Fluence, Gavita, Heliospectra). Older HPS fixtures at 1.7 µmol/J should be replaced before sizing a PV system — the energy savings often pay for the LED retrofit inside 18 months.

Worked example — 1000 sqft polycarbonate greenhouse in Ohio

A homesteader’s 1000 sqft (92.9 m²) polycarbonate hobby greenhouse near Columbus, Ohio:

• Glazing: twin-wall polycarbonate, U = 3.5 W/m²·K
• Envelope: 185.8 m²
• Climate: moderate, HDD18 = 2800 K·day
• Heating: cold-climate heat pump, average COP 2.8
• Lighting: 80 W/m² of LED, 4 hr/day, 180 days/yr
• Fans + circulation: 5 kWh/m²/yr

Computations:

• Heat thermal: 3.5 × 185.8 × 2800 × 24 / 1000 = 43,712 kWh/yr
• Heat electric: 43,712 / 2.8 = 15,611 kWh/yr
• Lighting: (80 × 92.9 / 1000) × 4 × 180 = 5,351 kWh/yr
• Fans: 92.9 × 5 = 465 kWh/yr
• Total electric: 21,427 kWh/yr
• Annual bill at $0.13/kWh: $2,785
• PV size: 21,427 / (5.0 × 365 × 0.78) = 15.05 kW
• PV cost at $2,000/kW: $30,100
• Simple payback before any verified incentive: 30,100 / 2,785 = 10.8 years

Common sizing mistakes

• Forgetting the envelope multiplier. A 100 sqft floor doesn’t have 100 sqft of glazed surface — it has roughly 200 sqft once you count sidewalls, endwalls, and the roof. Skipping the multiplier under-sizes heat load by half.
• Confusing thermal kWh with electric kWh. Heat pump output is the thermal load divided by COP. A 40,000 kWh thermal load needs only ~14,000 kWh of electricity if your heat pump is COP-2.8.
• Sizing PV to gas-fired heating. If you burn gas or propane, the heater isn’t electric, so PV won’t offset it. Set COP to 0 in the calculator and run a separate fuel cost analysis.
• Ignoring fan loads. They look small (5 percent of total) but circulation fans run year-round, including July when the heat pump is off. PV nameplate has to cover them.
• Net-metering arbitrage. In states with 1:1 net metering (NY, MA, CT, NJ), the simple payback math holds. In states with net billing at wholesale rate (CA NEM 3.0, AZ), you need ~30 percent more PV plus battery storage to recover the same economics. Check the latest tariff before sizing.

Stacking US incentives in 2026

• Federal tax treatment — do not use Form 5695 for a new post-2025 homeowner Section 25D expenditure. Business greenhouse PV or storage may be analysed under Section 48E on Form 3468, subject to the current commercial rules.
• USDA REAP grant — covers 50% of installed cost up to $1M for agricultural greenhouses. Application window opens twice yearly.
• State agricultural greenhouse efficiency programs — verify each program’s current funding, eligible costs and interaction with any USDA or business tax benefit before modelling a combined incentive stack.
• USDA NRCS EQIP — up to $300,000 for greenhouse efficiency retrofits including high-performance glazing and demand-controlled ventilation.
• MACRS 5-year depreciation — full bonus depreciation applies through 2026, then 40% in 2027.

An eligible agricultural business may reduce a $75,000 package through a verified REAP award, state assistance and applicable business tax treatment, but those benefits are not automatic and are not a homeowner Section 25D credit. Model the gross $75,000 first and add each confirmed benefit separately.

Sources

• ASHRAE 2020 Greenhouse Engineering Handbook — glazing U-values and design heat loads
• NREL PVWatts — peak sun hours by ZIP code
• USDA Rural Energy for America Program (REAP)
• ATTRA Sustainable Agriculture — Greenhouse Glazing Guide — 2023 economic comparison of greenhouse coverings
• Cornell CALS Controlled Environment Agriculture — LED lighting recommendations
• EnergySage 2026 Solar Marketplace Report — installed PV cost per watt
• DOE Building America — Cold-Climate Heat Pump Field Trials — heat pump COP at low ambient