Greenhouse Heat Loss & Heater Sizing Calculator
Calculate a greenhouse's heat loss from its surface area, glazing U-value (single glass, double poly, polycarbonate), wind exposure and the temperature difference between inside setpoint and the coldest night, size the heater in BTU/h and kW, and estimate seasonal fuel use and cost from heating degree days.
How it works
Q = U × A × ΔT × wind factor + 0.018 × volume × ACH × ΔT; heater = Q × 1.25; season BTU = (UA × wind + 0.018 × V × ACH) × 24 × HDD; fuel = BTU ÷ (BTU per unit × efficiency)
A greenhouse is almost all glazing, and glazing insulates poorly — single poly or glass loses about 1.1–1.2 BTU per hour per square foot per degree, double inflated poly and twin-wall polycarbonate roughly 0.65–0.7, so the design heat loss is the total covered surface times the U-value times the temperature difference on the coldest night, plus the air that leaks in and out. Sizing the heater to that loss with a modest margin keeps plants alive on the design night; multiplying the same coefficient by the season's heating degree days at the setpoint predicts fuel use and shows why a second poly layer or a night curtain pays back in one winter and why a lower night temperature is the cheapest saving of all.
Worked example
A 20 × 12 ft hoop house with 5 ft sidewalls and a 9 ft ridge has about 656 ft² of covering; under double poly (U 0.7) on a moderately windy site, holding 55 °F against a 10 °F night loses about 23,400 BTU/h including infiltration, so a 30,000 BTU/h heater is right. Over a 3,500 degree-day season that is roughly 44 MMBTU — about 600 gallons of propane, $1,550.
Frequently asked questions
What night temperature should I hold?
Frost protection only: 35–40 °F. Cool crops (lettuce, brassicas): 45–50. Tomatoes, peppers, cucumbers: 60–65. Every 5 °F costs roughly 10–20% of the fuel bill in a cold climate.
Does solar gain offset the loss?
By day, yes — greenhouses overheat and need venting; the design loss is for night, when there is none. Thermal mass (water barrels, a concrete floor) stores a few hours of daytime gain and trims the night load by 10–20%.
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Rules of thumb from extension-service and grower literature (UVI aquaponics, Cornell/Penn State mushroom guides, hydroponic nutrient manuals). Crops, cultivars, water chemistry and climate change the numbers; start conservatively and adjust from your own measurements.