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Solar Greenhouse Heating Calculator

Enter your greenhouse size and glazing type — get peak heat loss, solar collector area, water barrel thermal mass, and annual savings vs propane.

Greenhouse dimensions
ft
ft
ft
Glazing & temperatures
°F
°F
PSH
Greenhouse heating system sizing
19,034 BTU/hr peak heat loss
Floor area96 sqft
Total glazed surface426 sqft
Active solar collector area282 sqft
Thermal mass (water)240 gal (5 × 55-gal barrels)
Backup heater size5.6 kW electric
Solar panels (for backup heater)31 × 400W panels
Annual heating energy10,711 kWh/yr
Cost comparison
Annual propane cost (saved)$1,243/yr (444 gal)
Annual electric heating cost$1,392/yr
Passive solar system cost$769
Active solar system cost$61,190
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How to Use This Calculator

Enter your greenhouse dimensions

Measure the interior length, width, and average wall height of your greenhouse. Height matters because taller structures have more glazed surface area — and more surface area means more heat escaping in winter. A gutter-connect 12x24 at 10ft eave loses roughly twice the heat per foot of temperature difference as an 8x12 at 7ft.

Choose glazing and set temperature targets

Glazing R-value is the single biggest variable after greenhouse size. Single glass (R-0.9) loses heat 2.8x faster than triple-wall polycarbonate (R-2.5). For cold climates, upgrading glazing is almost always cheaper than adding more heating capacity. Set your target inside temperature based on crops: 35-40°F for frost protection, 50°F for cool-season crops, 60-65°F for tomatoes and peppers.

Read the results

The calculator outputs peak BTU/hr heat loss (worst-case night with coldest outside temp), active solar collector area, water barrel thermal mass, backup heater size, and a cost comparison between propane and solar-electric heating. Winter PSH determines how many solar panels you need to power an electric backup heater.

The Formula

Surface Area = Roof Area + Wall Area (all 4 walls) Heat Loss (BTU/hr) = Surface Area ÷ R-Value × Temperature Difference × 1.15 (infiltration) Annual BTU = Heat Loss × 16 hr/day × 120 heating days Solar Collector Area = Heat Loss ÷ (180 BTU/hr/sqft × PSH/8) Thermal Mass (water) = Floor Area × 2.5 gallons/sqft Backup Heater kW = Heat Loss ÷ 3,412 BTU/kWh Solar Panels = Annual kWh ÷ 365 days ÷ PSH ÷ 0.80 ÷ 0.4 kW/panel

The 1.15 infiltration multiplier accounts for air leaks at doors, vents, and glazing joints — typical for unweatherized greenhouses. The 180 BTU/hr/sqft collector output assumes a south-facing glazed air collector in average winter sun. Water thermal mass absorbs 1 BTU per gallon per degree F — 55-gallon barrels painted black and placed along the north wall are the simplest, lowest-cost thermal mass option.

Example

Sarah — Market garden 12x24 in Pennsylvania

Sarah runs a small market garden operation and wants to extend her growing season with a 12x24 twin-wall polycarbonate greenhouse. She wants to maintain 50°F inside when it's 20°F outside, with 4.0 winter PSH.

Greenhouse12 × 24 ft, 10 ft walls
GlazingTwin-wall polycarbonate R-1.7
Target inside50°F
Winter low20°F (ΔT = 30°F)

Result

Peak heat loss~11,200 BTU/hr
Solar collector area~62 sqft
Thermal mass720 gal (13 × 55-gal barrels)
Backup heater3.3 kW electric
Annual propane savings~$890/yr (320 gal)
Passive system cost~$900 (barrels + insulation)

Sarah's 12x24 polycarbonate greenhouse needs about 62 sqft of south-facing solar air collector and 13 water-filled 55-gallon barrels for thermal mass. The passive thermal mass system pays for itself in under a year versus propane heating. Adding a 3.3 kW backup electric heater powered by 3 solar panels handles the coldest nights.

FAQ

Passive solar uses the greenhouse structure itself as a solar collector — south-facing glazing admits sunlight, thermal mass (water barrels, concrete, stone) absorbs heat during the day and releases it at night. No moving parts, no electricity, no cost to operate. Works well if the greenhouse is properly oriented (long axis east-west) and has adequate thermal mass. Active solar uses dedicated solar air or water collectors, fans, and sometimes solar panels to power a backup electric heater. More complex but can handle colder climates and poorly oriented greenhouses. Most growers use a combination: maximize passive design first, add active backup for the coldest nights.
The rule of thumb is 2-3 gallons of water per square foot of floor space for climates with temperature swings of 20-30°F overnight. For example, a 12x24 (288 sqft) greenhouse needs 576-864 gallons — roughly 10-16 standard 55-gallon barrels. Place them along the north wall, painted flat black to maximize solar absorption. For larger temperature swings (more than 30°F), increase to 3-4 gallons per sqft. Water thermal mass is far more effective than rocks or concrete by volume — water holds 62.4 BTU/gallon/°F versus 20-25 BTU/cubic foot for masonry.
For climates below 0°F, triple-wall polycarbonate (R-2.5) is the clear winner: it retains 65% more heat than single glass, diffuses light to reduce hot spots, is nearly unbreakable, and costs $1.50-2.50/sqft less installed than glass. For moderate climates (10-30°F lows), twin-wall polycarbonate (R-1.7) is the best value — roughly half the heat loss of single glass at a fraction of the cost. Double poly film (two layers of greenhouse film with air space) achieves R-1.5 for the lowest material cost but requires replacement every 3-5 years. Single glass is best reserved for mild climates or historical/aesthetic greenhouses where the look matters.
Yes, but winter is the hardest time for solar — short days and low sun angles mean fewer peak sun hours. In most northern states (Zone 5-6), winter PSH averages 2.5-3.5 hours/day. To power a 3 kW backup heater running 6 hours/day in a cold climate, you'd need 8-12 solar panels plus battery storage or grid backup. The math works best when solar handles the mild shoulder season (spring/fall) and only supplements in deep winter, with a propane or electric backup taking over on the coldest nights. Solar air collectors (direct hot air from sunlight) are often more cost-effective for greenhouses than PV panels — they capture more heat per dollar in this application.
Temperature targets by crop type: 35-40°F — frost protection only (store dormant plants, root vegetables); 45-50°F — cool-season crops (spinach, lettuce, kale, chard, arugula — all grow well above 45°F); 55-60°F — herbs, some tomato varieties, strawberries; 65-70°F — tomatoes, peppers, cucumbers, basil. Each 5°F increase in target temperature roughly doubles heat loss when outside temps are below 20°F, so growing cool-season crops in a 50°F greenhouse uses roughly 1/4 the energy of growing tomatoes at 70°F in the same structure.

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