Home Greenhouse Heater Size & Operating Cost Calculator

Greenhouse Heater Size & Operating Cost Calculator

Q: How is greenhouse heater size estimated?

The calculator estimates conduction through the exposed covering, infiltration heat loss and perimeter heat loss using greenhouse heat-loss equations published by UGA Extension. It then shows the calculated design heat loss and a transparent planning band rather than presenting a single value as a guarantee.

Q: Why does glazing matter so much?

Greenhouse covering materials have very different resistance to heat flow. UGA Extension publishes overall R-values for single glass, double glass, single and double film, and double acrylic or polycarbonate assemblies, so the same greenhouse geometry can have a materially different heating load depending on the covering.

Q: What outside temperature should I enter?

Use a defensible cold-weather design temperature for your location and project, not today's temperature. The calculator does not determine local design weather or code requirements for you.

Q: Can this calculator size a gas heater safely?

It is a planning estimator, not a combustion or installation design. Fuel-fired greenhouse heaters require correct venting, combustion air, clearances and installation under the current manufacturer instructions and applicable local requirements.

Estimate greenhouse design heat loss from exposed surface area, covering R-value, air leakage, perimeter conditions and the indoor-to-outdoor temperature difference. Select a documented model to preload available specifications or enter your own greenhouse geometry.

Estimate greenhouse design heat loss

Select a documented greenhouse model to preload the footprint, peak height and covering where available, or enter your own dimensions. The sidewall/eave height remains a required geometry input because it is not consistently published across the current catalog and GreenhouseMatch does not invent it.

Documented greenhouse model (optional) A selected model preloads documented dimensions and a compatible covering preset where possible. Units Width Length Sidewall / eave height Required for the exposed-surface geometry. Verify this from the model drawing or your structure. Peak height Known exposed covering area (optional) If you have a manufacturer/drawing surface-area value, enter it here. It overrides the simple gable-envelope area estimate; use sq ft in imperial mode or m² in metric mode. Greenhouse covering Indoor target temperature Outside design temperature Use a cold-weather planning/design temperature appropriate to your location. Air leakage / construction condition Perimeter condition Site wind exposure Fuel / energy type for cost estimate Heater efficiency Enter delivered efficiency as a percent. Electric resistance is approximately 100% at the point of use; fuel-fired equipment varies. Energy price Price per kWh. Hours at design load Used only for the design-load cost scenario; not a seasonal bill forecast.

Results update automatically when the required geometry, covering and temperatures are present.

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What this calculator includes

Greenhouse heating is an envelope problem, not a floor-area problem. UGA Cooperative Extension breaks winter heat loss into conduction through the greenhouse covering, infiltration from outside air and perimeter heat loss. This calculator follows that structure: it estimates exposed wall and roof area from a simple gable geometry, applies the published covering R-value, estimates air volume for infiltration and adds perimeter loss.

Component	Planning equation	Why it matters
Conduction	Q = A × ΔT ÷ R	More exposed area, a larger temperature difference or a lower-R covering raises heat loss.
Infiltration	Q = 0.02 × V × C × ΔT	Air leakage can become a meaningful load, especially in older or poorly sealed structures.
Perimeter	Q = P × L × ΔT	The greenhouse-to-ground edge adds heat loss; UGA publishes separate insulated and uninsulated coefficients.

The result is a planning estimate at the entered temperature difference. It does not account for every construction detail, thermal bridge, foundation design, curtain system, internal mass, snow cover, solar gain or local design requirement.

Why exact geometry matters

Two greenhouses with the same nominal floor area can require different heater capacities. A taller greenhouse has more wall area and more air volume. A steeper gable has more roof covering. A double-wall polycarbonate or double-glass assembly loses less heat through the covering than single film or single glass under the published UGA values.

For a gable greenhouse, GreenhouseMatch calculates the sloped roof length from half the width and the rise from eave to peak. That is why the calculator asks for both eave and peak height instead of multiplying floor area by a generic factor.

Compare greenhouse heaters after sizing the load

Use the calculated BTU/h requirement as a planning starting point, then compare heater output, controls, fuel type, venting requirements and manufacturer installation instructions.

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Interpreting the result

Planning for a long heating season? Pair this calculation with the Greenhouse for Cold Climates guide. If the purpose is plant storage or continuous winter production, also see overwintering plants and year-round greenhouse planning so the temperature target matches the actual use case.

The calculator shows the calculated design heat loss and a planning capacity band from the calculated load up to 20% above it. The upper figure is not a code-required safety factor. It is simply a transparent planning allowance so the result does not imply that a heater rated exactly at the theoretical loss will necessarily maintain temperature under every real-world condition.

UGA's worked greenhouse example also notes that an exposed windy site could require about 20% more heat. That is why wind exposure is a separate optional adjustment instead of being silently added to every calculation.

Operating cost is a design-load scenario

If you enter an energy price, the calculator converts the heat requirement into electricity, natural-gas therms or propane gallons using published heating values and your entered heater efficiency. The cost assumes the greenhouse remains at the entered design load for the selected number of hours. Real seasonal consumption will vary with weather, thermostat settings, solar gain, night curtains, air leakage and equipment cycling.

UMass Amherst emphasizes that heater efficiency, maintenance, make-up air and controls can materially affect greenhouse energy use. Fuel-fired equipment also needs adequate combustion air and safe venting; this calculator does not replace manufacturer instructions or a qualified installer.

Covering R-values used

Covering	Overall R-value used
Single glass	0.91
Double glass, ¼-inch space	2.00
Single polyethylene / film	0.83
Double separated film	1.43
Double acrylic or polycarbonate	2.00

These are UGA Extension's published overall assembly values, including surface coefficients. A manufacturer's product-specific tested thermal value can be preferable when it is directly applicable to the exact greenhouse assembly.

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Frequently asked questions

How is greenhouse heater size estimated?

The calculator combines covering conduction, infiltration and perimeter heat loss using the entered geometry and temperature difference. It reports a planning load, not a guarantee.

Why does glazing matter so much?

The published resistance to heat flow differs substantially between single-layer and double-layer assemblies, so covering choice can materially change the calculated load.

What outside temperature should I enter?

Use an appropriate cold-weather design temperature for the location and project. Do not use today's temperature merely because it is convenient.

Can this calculator size a gas heater safely?

It can estimate required heat output, but combustion air, venting, clearances, fuel supply and installation must follow the exact heater instructions and applicable requirements.