Greenhouse Heating Cost Calculator

Greenhouse heating demand is a steady-state conduction problem. Heat leaves through the glazed envelope at a rate equal to the exposed surface area multiplied by the U-value of the covering and by the inside-to-outside temperature difference, in units of BTU per hour per square foot per degree F. Published U-values for common coverings span single polyethylene film at about 1.20 and single glass at 1.13 down to double poly at roughly 0.70, double glass at 0.65, twin-wall 8 mm polycarbonate near 0.58 and triple-wall 16 mm polycarbonate near 0.36 - a spread of more than three to one between the worst and best options. A separate allowance of roughly 25 percent is added for infiltration, the uncontrolled exchange of warm inside air for cold outside air through door seals, vent gaps and fan shutters, since that loss is driven by air changes rather than conduction. Fuel is then derived by dividing the hourly load by the heating value of the fuel and the burner efficiency: about 100,000 BTU per therm of natural gas, 91,500 per gallon of propane, 138,500 per gallon of heating oil and 3,412 per kWh of electricity, at roughly 80 percent efficiency for vented combustion units and effectively 100 percent for resistance electric. Heaters are normally sized on the coldest design temperature for the location rather than a seasonal average, with a further 25 percent margin so the system can recover after venting. The seasonal cost figure is inherently rougher than the peak load figure, because it multiplies a single average temperature difference across a whole season instead of integrating hourly weather; degree-day methods give better seasonal estimates. The model also ignores solar gain, which can eliminate daytime heating entirely on clear days, and ignores latent heat carried off by transpiration and by moisture condensing on the glazing, so it tends to overstate daytime demand and understate the value of night-time energy curtains, which typically cut night heat loss by 25 to 50 percent.

Growing plants under cover is ancient - Pliny the Elder records that the emperor Tiberius was supplied year-round with a cucumber-like vegetable raised in wheeled beds glazed with sheets of translucent mineral known as specularia. Heated glasshouses proper appeared in early modern Europe as orangeries built to overwinter citrus for aristocratic collections, and the technology matured in nineteenth-century Britain when cheap sheet glass and iron framing made large structures possible; Joseph Paxton, who built the great conservatory at Chatsworth in the 1830s, went on to design the Crystal Palace for the Great Exhibition of 1851. These houses were heated first by flues and stoves and then by hot-water pipe circuits, a distribution method still preferred for high-value crops because it warms uniformly at bench level. The economics that make this calculation matter arrived with plastic. Emery Myers Emmert, a horticulturist at the University of Kentucky, pioneered the use of polyethylene film as a greenhouse covering in the 1950s, cutting construction cost so far that commercial greenhouse acreage expanded rapidly. Fuel was cheap enough that heat loss was an afterthought until the oil shock of 1973, after which double-layer inflated poly, thermal blankets and perimeter insulation were adopted across the industry specifically to cut fuel bills. The engineering method used here - envelope area times U-value times temperature difference, plus an infiltration allowance - is the standard steady-state heat loss calculation formalised in the handbooks of the American Society of Heating, Refrigerating and Air-Conditioning Engineers, whose predecessor societies date to 1894, and it was adapted to greenhouses in cooperative extension bulletins from the 1970s onward.