Climate Control Energy Cost Calculator

Heat escapes (or enters) through the entire greenhouse covering — walls and roof — not just the ground footprint, so using floor area alone would understate the actual heat-transfer surface significantly. The roughly 2.5x floor area approximation is a common rule of thumb for typical greenhouse shapes, though a greenhouse with an unusually tall or steeply pitched roof would have a somewhat higher actual ratio.

Both are cumulative annual measures of how far and how long outdoor temperature deviates from a 65°F baseline — heating degree days accumulate on cold days below that baseline, cooling degree days accumulate on warm days above it. A location with cold winters and mild summers will show high HDD and low CDD, while a hot climate shows the reverse, and both figures are widely published by weather services for most locations.

U-value multiplies directly into both the heating and cooling BTU calculations, so a lower U-value (better-insulated covering, like double-wall polycarbonate versus single-layer poly film) reduces both heating and cooling energy proportionally. Since it scales the entire heat-transfer calculation, upgrading glazing is often one of the highest-leverage changes for reducing a greenhouse's total annual energy cost.

The BTU figure represents the actual heat that needs to be replaced inside the greenhouse, but no furnace or boiler converts 100% of its fuel into usable heat — some energy is lost as exhaust or through inefficiencies in the system. A less efficient heating system has to burn more total fuel to deliver the same amount of usable heat, which is why the therms figure divides raw BTU loss by efficiency rather than just the raw BTU number.

Running the calculator once with your current glazing U-value and system efficiency, then again with an upgraded U-value or higher-efficiency equipment, lets you compare the resulting cost-per-square-foot figures directly to estimate the annual savings from that specific improvement. That savings figure can then be weighed against the upgrade's upfront cost to judge payback time.