Greenhouse Heating Cost Calculator
Calculate greenhouse heating costs from volume, target temperature, and energy source. Enter values for instant results with step-by-step formulas.
Formula
Heat Loss (BTU/hr) = Surface Area x U-value x Delta-T x 1.25 (infiltration)
Total heat loss is calculated from the greenhouse surface area (walls + roof) multiplied by the covering material's U-value and the temperature difference between inside and outside. A 25% infiltration factor accounts for air leakage through gaps and openings.
Worked Examples
Example 1: Small Hobby Greenhouse - Natural Gas
Problem: Calculate heating costs for a 20x12x8 ft greenhouse with double poly covering, target 60F, outside 25F average, using natural gas at $1.20/therm, 14 hours/day for 120 days.
Solution: Floor area = 20 x 12 = 240 sq ft\nWall area = 2(20x8) + 2(12x8) = 512 sq ft\nRoof area = 240 x 1.15 = 276 sq ft\nTotal surface = 788 sq ft\nU-value (double poly) = 0.70\nDelta T = 60 - 25 = 35F\nHeat loss = 788 x 0.70 x 35 = 19,306 BTU/hr\nWith infiltration (+25%) = 24,133 BTU/hr\nGas per hour = 24,133 / (100,000 x 0.80) = 0.30 therms\nDaily = 0.30 x 14 = 4.22 therms = $5.07\nSeason = $5.07 x 120 = $608
Result: Season cost: ~$608 | $5.07/day | Heater size: ~30,000 BTU needed
Example 2: Commercial Greenhouse - Propane
Problem: Calculate costs for a 60x30x12 ft greenhouse with single poly, target 70F, outside 20F, propane at $2.50/gallon, 18 hours/day for 180 days.
Solution: Floor area = 1,800 sq ft\nWall area = 2(60x12) + 2(30x12) = 2,160 sq ft\nRoof area = 1,800 x 1.15 = 2,070 sq ft\nTotal surface = 4,230 sq ft\nU-value (single poly) = 1.20\nDelta T = 70 - 20 = 50F\nHeat loss = 4,230 x 1.20 x 50 = 253,800 BTU/hr\nWith infiltration = 317,250 BTU/hr\nPropane/hr = 317,250 / (91,500 x 0.80) = 4.33 gal\nDaily = 4.33 x 18 = 78 gal = $195\nSeason = $195 x 180 = $35,100
Result: Season cost: ~$35,100 | $195/day | Heater size: ~400,000 BTU needed
Frequently Asked Questions
How do I calculate the heating requirements for my greenhouse?
Greenhouse heating requirements are determined by calculating the total heat loss through the structure, which depends on three primary factors: the total surface area exposed to outside air, the thermal conductivity of the covering material (expressed as a U-value), and the temperature difference between the desired inside temperature and the coldest expected outside temperature. The basic formula is Heat Loss (BTU/hr) = Surface Area (sq ft) x U-value (BTU/hr/sqft/F) x Temperature Difference (F). You must also add approximately 25% for air infiltration losses through gaps, vents, and door openings. The resulting BTU per hour figure tells you the heating capacity needed. For sizing a heater, add another 25% safety margin beyond the calculated heat loss to ensure the system can handle extreme cold snaps and recovery after ventilation. Most growers calculate based on the coldest expected nighttime temperature in their region rather than the average winter temperature.
Which greenhouse covering material provides the best insulation?
Multi-wall polycarbonate panels offer the best insulation among common greenhouse coverings, with 16mm triple-wall panels achieving U-values as low as 0.36 BTU/hr/sqft/F, which is roughly three times more insulating than single-layer glass at 1.13 BTU/hr/sqft/F. Double-layer polyethylene film with an air gap is a popular cost-effective option with a U-value around 0.70, providing nearly double the insulation of single-layer poly at a fraction of polycarbonate cost. Double-pane glass at 0.65 is similar to double poly but significantly more expensive and heavier, requiring stronger framing. Fiberglass panels at 0.83 offer moderate insulation with excellent light diffusion. The choice involves trade-offs between insulation value, light transmission, durability, and cost. Many commercial growers use double poly inflated with a small blower fan because it provides good insulation at very low material cost, even though it must be replaced every three to four years.
What is the most cost-effective fuel source for greenhouse heating?
The most cost-effective fuel depends heavily on local prices and availability, but natural gas is generally the cheapest option in areas with pipeline access, typically costing 40 to 60 percent less than propane or heating oil per BTU delivered. Electric heating is the most expensive per BTU in most regions but offers 100% efficiency and requires no flue or ventilation for combustion gases. Propane is widely available and moderately priced but requires storage tanks and regular deliveries. Wood pellet and biomass boilers offer very low fuel costs in areas with cheap biomass supply and can reduce heating costs by 50 to 70 percent compared to propane, though they require significant upfront investment and more labor for operation. Some innovative growers use waste vegetable oil, geothermal heat pumps, or solar thermal systems to supplement conventional heating. A growing number of operations combine heat pump technology with backup gas heating to optimize costs throughout the temperature range.
How can I reduce greenhouse heating costs without changing fuel sources?
Several strategies can significantly reduce greenhouse heating costs without switching fuel sources. Adding an inner layer of polyethylene or bubble wrap as a thermal curtain reduces heat loss by 30 to 40 percent, especially when deployed at night over the crop zone. Installing energy curtains that automatically retract during the day and deploy at night can save 25 to 50 percent on heating costs. Sealing air leaks around doors, vents, fan shutters, and foundation joints can reduce infiltration losses by 15 to 25 percent. Using perimeter insulation of rigid foam board on the inside of knee walls below bench height reduces foundation heat loss with minimal impact on light. Locating heating pipes or ducts below benches rather than overhead puts heat where plants need it most. Grouping heat-loving crops together in the warmest zones and allowing cool-tolerant crops to grow in cooler areas reduces the average temperature requirement. Finally, using thermal mass such as water barrels or concrete floors absorbs solar heat during the day and releases it at night, reducing peak heating demand.
What size heater do I need for my greenhouse?
Proper heater sizing is critical because an undersized heater cannot maintain target temperatures during cold spells, while an oversized unit wastes fuel through short-cycling and uneven heating. Calculate the maximum heat loss at your design temperature, which should be the coldest temperature expected in your area based on historical weather data for the coldest 1 percent of winter hours. Add 25 percent for air infiltration and another 25 percent as a safety factor, giving you the required heater output in BTU per hour. For a 30x20 foot greenhouse with double poly covering, targeting 65 degrees when it is 30 degrees outside, the heat loss is approximately 15,000 to 20,000 BTU per hour, so a 25,000 BTU heater would be appropriate. For larger greenhouses, multiple smaller heaters distributed throughout the space provide more uniform temperature than one large unit. Forced-air unit heaters are the most common and cost-effective choice, while hot water systems with in-floor or under-bench piping provide superior uniformity for high-value crops.
How do greenhouse gases trap heat?
Greenhouse gases (CO2, methane, N2O, fluorinated gases) absorb and re-emit infrared radiation, warming the atmosphere. Global Warming Potential (GWP) compares gases to CO2 over 100 years: methane has a GWP of 28, N2O is 265. Total forcing is measured in watts per square meter and currently exceeds 3 W/m^2.