Greenhouse Climate Calculator
Estimate the heating energy cost of a greenhouse in its coldest month from its size, climate zone, target temperature, and glazing insulation. Use it when budgeting a new greenhouse build or evaluating energy-saving retrofits.
Last updated: September 2026
Formula below · 2 sources (usda.gov, Wikipedia) · Updated Sep 2026
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About this calculator
Greenhouse heating is driven by heat loss through the glazing: Q = U × A × ΔT, the standard heat-loss formula used in extension greenhouse guides. U is the glazing's heat-transfer coefficient in BTU per hour per square foot per °F (about 1.1 for single glass or single poly, 0.7 for double poly or twin-wall polycarbonate, and about 0.45 for well-insulated structures). A is the glazed surface area, taken here as 1.8 × the floor area, typical of gable and hoop houses. ΔT is the target inside temperature minus the average outdoor temperature of the coldest month for your zone. Monthly heat = U × A × ΔT × 720 hours; dividing by 3,412 BTU per kWh and multiplying by your $/kWh gives the cost of electric resistance heat. Formula: Monthly Cost = greenhouseSize × 1.8 × U × max(0, targetTemp − outdoorTemp) × 720 ÷ 3,412 × energyCost. The model ignores daytime solar gain (which lowers the bill) and air leakage (which raises it, often 10-30%), and it does not estimate summer cooling; for gas or propane heat, convert your fuel price to an equivalent $/kWh (1 therm = 29.3 kWh) and divide by the heater efficiency.
How to use
Example: a 2,000 sq ft double-wall greenhouse (U = 0.7) in zone 7-8 (coldest-month average about 42°F) heated to 70°F, with electricity at $0.12/kWh. Step 1: glazed area = 2,000 × 1.8 = 3,600 sq ft. Step 2: ΔT = 70 − 42 = 28°F. Step 3: monthly heat = 3,600 × 0.7 × 28 × 720 = 50,803,200 BTU. Step 4: 50,803,200 ÷ 3,412 = 14,889.6 kWh × $0.12 = $1,786.75 for the coldest month. The same house with single-wall glazing (U = 1.1) would cost about $2,807.75, and in zone 5-6 (29°F) the double-wall house would cost about $2,616.31.
Frequently asked questions
How does insulation level affect greenhouse heating costs?
Heat loss is proportional to the glazing's U-value, so lower is better: single-layer glass or polyethylene loses about 1.1 BTU per hour per square foot per °F, double-layer poly or twin-wall polycarbonate about 0.7, and well-insulated structures about 0.45. Moving from single to double glazing cuts the heating bill by about 36% in this calculator (0.7 ÷ 1.1), in line with the 30–40% savings growers typically report. Thermal curtains drawn at night cut losses further. The payback period on better glazing is often two to four heating seasons.
What climate zone should I enter for my greenhouse location?
Use your USDA Plant Hardiness Zone. Lower zone numbers are colder (zone 3-4 covers places like Minnesota and North Dakota; zone 9-10 covers Florida, southern California and southern Arizona). The calculator uses a typical average outdoor temperature for the coldest month in each zone band (about 12°F, 29°F, 42°F, 57°F and 68°F). If you know your own January average temperature, the result will be more accurate if you pick the band closest to it. Moving one band colder raises heating cost substantially because the temperature difference grows.
Why do greenhouse energy costs vary so much between summer and winter months?
The temperature differential between inside and outside the greenhouse drives energy demand. In winter, growers must add heat to offset large inside-to-outside temperature gaps, which can be 40–60°F in northern climates. In summer, the challenge reverses: solar gain superheats the interior, and ventilation or active cooling is needed to prevent crop stress. Energy costs can swing by a factor of three to five between peak heating months and moderate shoulder seasons. Running this calculator with season-specific temperature targets helps you forecast full-year energy budgets accurately.