How to Size a Greenhouse Heater for Cold Nights
Greenhouse heating guide
A greenhouse heater is sized against the heat that escapes on a cold night, not simply against floor area. The estimate depends on the indoor temperature you want, a realistic outdoor design temperature, the exposed envelope, the thermal performance of its surfaces and air exchange. This guide shows how to calculate an approximate peak heating load, where assumptions enter and why that figure differs from an energy bill. Use it to compare documented heater output for a defined greenhouse and weather condition—not as a promise that every corner will hold one temperature.
Set the overnight temperature you need
First decide what the heater must achieve. Holding a greenhouse just above a chosen threshold while plants rest is a different job from maintaining conditions for active growth. The appropriate threshold depends on the plants and the purpose of the space; use reliable, plant-specific horticultural guidance rather than assuming one setting suits everything. Record a target air temperature for the coldest part of the night, not a daytime average or a vague label such as “frost protection”. The higher that target is above the outside temperature, the greater the heat loss the heater must replace. For a mixed planting, base the target on the group whose needs govern your decision.
Treat the target as an air reading, not a guarantee that every leaf, pot or shelf will reach the same temperature. A sensor near the heater may show a different value from a low corner, draughty doorway or far end. Place a thermometer or controller sensor where it represents the plants you intend to protect, following its maker’s instructions, and compare overnight minimums across the growing area. If a small temperature shortfall could harm valuable plants or plants that tolerate only a narrow temperature range, do not rely on a rough estimate alone; get more detailed design advice and monitor conditions in use.
Choose a cold-night outdoor temperature
Choose the outside temperature you want the design to withstand, rather than using the forecast for one night. A long-term average winter minimum may be too mild for a cold snap, while the all-time record low could produce a peak-load figure for an event you do not intend to cover. Use local climate records as a starting point, then consider the site’s exposure and how often you need the target maintained. Australian conditions vary between locations and individual sites, so a broad national average is not a useful design value. Terrain, elevation and shelter can also make a nearby weather station an imperfect match.
Subtract the selected outside value from the indoor target to get ΔT, the temperature difference used in the basic calculation. For example, an indoor target of 5°C and a chosen outdoor value of −5°C give a 10°C difference. Write this as 5 − (−5) = 10 to avoid losing the minus sign when entering the figures. Keep the two original temperatures alongside the difference: a difference of 10 degrees does not, by itself, show which indoor target or outdoor condition you selected. Wind can increase leakage and heat transfer at exposed surfaces; on clear, calm nights, some surfaces may also cool below the measured outdoor air temperature. A simple air-to-air estimate may miss part of that load. State the weather assumption plainly, and seek a more detailed assessment if maintaining the target is critical.
Measure the exposed surface area
List every boundary separating the heated space from outdoors or from a colder space: roof planes, side walls, end walls, doors and any unheated adjoining section. Measure the actual surface dimensions. For a simple rectangular panel, length multiplied by height gives its area; measure a sloping roof along the slope, not from the building’s plan view. A triangular gable has an area of half its base multiplied by its height. Keep surfaces with different construction separate, and use dimensions that match the area convention in the technical data. A measurement sheet can have one row for each surface, with columns for dimensions, area, construction and the temperature on the other side. Where a door is calculated separately, subtract its area from the surrounding wall. This keeps the later calculation traceable and prevents the door area being included under two different construction values.
An area listing is not necessarily the surface area needed for heat-loss calculations. Bloomcabin’s Spacious Greenhouse is listed from 47 to 70 m²; for the selected size, use its actual roof and wall dimensions rather than treating the listed area as a heater rating. In an attached or partitioned layout, count only boundaries facing outdoors or a genuinely cooler space, and identify which side of a shared wall belongs to the heated zone. Do not count a surface twice because it appears in more than one drawing.
Use documented U-values for each surface
For each exposed surface, estimate heat loss in watts as U × A × ΔT. U is the thermal transmittance in W/m²·K, A is the surface area in m² and ΔT is the inside-to-outside difference in kelvins. A difference of 1°C has the same size as 1 K. Add the result for each roof, wall, door and other boundary using its own U-value. A lower U-value means less heat passes through a surface for the same area and temperature difference. If the other side of a boundary is not outdoors, use the appropriate temperature difference rather than automatically applying the outdoor value.
Use U-values documented for the exact glazing and construction, or ask a building-services professional to establish an appropriate value. A product name or photograph does not tell you the construction details or the whole-surface U-value. The Aluminium Classic Greenhouses listing covers sizes from 3.77 to 28.08 m²; identify the chosen version and obtain relevant technical data before applying a coefficient. If a reliable value is unavailable, mark the estimate as uncertain rather than borrowing a number from a different greenhouse.
Estimate heat loss from air exchange
Air escaping through joints or entering through gaps also has to be heated. A rough estimate of sensible heat loss is Qair ≈ 0.33 × n × V × ΔT watts, where n is air changes per hour, V is internal volume in m³ and ΔT is the temperature difference in kelvins. The factor 0.33 is an approximate heat-capacity term for air in compatible units; it is not a measured property of the greenhouse. Estimate volume from the inside dimensions and use a more exact shape where a sloping roof, partition or adjoining section makes a simple rectangular approximation misleading.
The difficult input is n: a catalogue footprint or greenhouse name does not reveal the real air-change rate, which depends on joints, doors, vents and wind. The Wall Greenhouse range is listed from 3.69 to 14.34 m², but its area alone says nothing about leakage or the air exchange of a particular installation. Use a documented figure when available, have the enclosure assessed, or calculate a range using clearly stated assumptions. If your air-change estimate includes planned ventilation, do not add that same airflow again as a separate loss.
Include floors, doors and the ventilation plan
Treat the floor and ground contact separately from the roof and walls. Heat flow to soil depends on the ground temperature and construction, so multiplying floor area by a glazing U-value would not be appropriate. Frames, bases and junctions can also add losses, but a quick calculation should not invent a universal allowance for them. Include a documented value or professional estimate where possible; otherwise, record the omitted terms so it is clear what the calculation leaves out.
Define the operating state you are sizing for. If vents or doors will be open at night, estimate the heat loss for that airflow rather than for a closed greenhouse. If they will stay closed, that does not mean incidental leakage is zero. The Limited Edition Aluminium Greenhouses listing covers sizes from 3.77 to 28.08 m², but those figures do not quantify losses at the floor, door edges or junctions. Use the chosen layout and intended overnight practice, and follow the heater maker’s and a qualified installer’s guidance on any required ventilation and safe operation.
Add the terms in a worked example
With surface values and an air-change estimate, add the terms: Qtotal ≈ Σ(U × A × ΔT) + 0.33 × n × V × ΔT, then include any separately quantified ground or other losses. The Σ means to repeat the surface calculation for every boundary and add the results. For arithmetic only—not a typical greenhouse specification—assume 45 m² of exposed surface with a hypothetical U-value of 5.0 W/m²·K, a 25 m³ internal volume, n = 1.0 air changes per hour and ΔT = 10 K. The surface calculation is 5.0 × 45 × 10 = 2,250 W; air exchange adds 0.33 × 1.0 × 25 × 10 = 82.5 W, or about 83 W. The single surface term works here because the example assigns one U-value and temperature difference to the entire exposed area. For a greenhouse with several surface types, calculate each row separately before adding them. Keep the air-exchange subtotal separate too, so you can revise either input without rebuilding the whole calculation.
Together, those terms give about 2,333 W, or 2.33 kW, before any separately assessed ground, junction or night-sky effects. These inputs are deliberately hypothetical and do not describe a Bloomcabin product or a recommended heater. For a selected structure, use its actual dimensions and documented values instead. The T-Model Orangery Greenhouse listing covers sizes from 16.52 to 30 m²; that area range is not a substitute for surface measurements or a tested heat-loss figure. Keep the assumptions beside the result so it is clear what has been included.
Match the result to a heater’s rated heat output
The total is an estimate of peak heat loss for the chosen indoor and outdoor temperatures, not an appliance recommendation by itself. Compare it with a heater’s documented heat output in watts or kilowatts, using output rather than fuel input, electrical consumption or a marketing claim about the area it can heat. One kilowatt equals 1,000 watts. Confirm that the rating applies at the conditions in your calculation and check any limits the manufacturer gives for controls or use. If only an input or fuel-use figure is published, ask the supplier or a qualified heating professional for the usable heat output.
Avoid converting the estimate into a universal allowance, such as a fixed number of watts per square metre or an automatic percentage uplift. An allowance may be appropriate when inputs are uncertain, but it depends on which losses were omitted and how much risk of a cold excursion you accept. The Pyramid Greenhouse is listed in sizes from 5.53 to 18.9 m²; that range is no basis for applying one heater rating across every size. Check the selected version’s technical information and consider how the heat will be controlled across the space. Confirm equipment suitability and installation with a qualified professional.
Keep heater capacity separate from running cost
Capacity, expressed in kW, is the rate at which heat can be supplied; energy use is measured over time in kWh. A heater cycling under thermostat control may not run at full output every hour, while a colder night, higher target or greater air leakage can make it operate for longer. Estimate running cost separately using expected operating hours, actual energy input or fuel use, relevant appliance efficiency and the tariff that applies to your property. Do not multiply the peak heat demand by every winter hour and present the result as a bill forecast.
Define the heated zone as well. Bloomcabin’s Limited Edition Aluminium Greenhouses with Storage are listed in sizes from 5.52 to 28.08 m²; when comparing a layout with a storage section, establish which spaces are intended to share the setpoint before estimating the heat load or run time. Do not assume from the name alone that an adjoining section is heated, insulated or isolated. The cost calculation also needs local tariff information and heater-specific input data; a greenhouse area range supplies neither.
Check uncertainty in the heating estimate
A basic calculation does not model every way a greenhouse responds over time. Sun-warmed surfaces and stored heat can reduce demand for a while; wind, night-sky radiation, leaks, an opened door or a local cold pocket can increase it. Conditions may vary across a long or partitioned structure. The formula also does not predict thermostat cycling or how quickly the greenhouse cools after a power interruption. Treat the result as an estimate for stated conditions, not as a promise of the same minimum temperature in every location and every weather event.
Before choosing a heater, write down the target temperature, cold-night value, measured surface areas, documented U-values, air-exchange assumption, omitted losses and rated heat output. Ask which technical information applies to the precise size and configuration. The Bloomcabin Prestige Greenhouse is listed from 3.77 to 28.08 m²; its listed area range does not establish a heating requirement, insulation or leakage. To see which uncertainties matter most, repeat the calculation with a clearly labelled alternative for one uncertain input at a time. Keep the other inputs unchanged so you can see how much that assumption affects the total. Record the original and revised results together, rather than replacing the first estimate and losing its basis. This comparison distinguishes an input that barely changes the result from one that could affect the capacity you choose; it does not establish which assumed value is correct. If the result is especially sensitive to an unknown input, resolve that uncertainty or involve a qualified heating or building professional. Once in use, overnight temperature records can show how well the assumptions match actual conditions.
Frequently asked questions
Can I size a greenhouse heater from floor area alone?
No. Floor area does not reveal the total roof and wall area, the heat-transfer values of those surfaces or how much air escapes. Two greenhouses with similar footprints can have different heat losses. Use dimensions for the chosen structure, documented construction data, an overnight target and an air-exchange estimate.
What outdoor temperature should I use?
Choose a cold-night design temperature that reflects your local conditions and how often you need the greenhouse to stay at its target. Local climate records are a useful starting point, but site exposure can matter. An average night may be too mild; an all-time record may be more extreme than your design needs.
What does the 0.33 air-loss factor mean?
It is an approximate heat-capacity factor for air. In the formula, use air changes per hour for n, internal volume in cubic metres for V and a temperature difference in kelvins for ΔT; the result is an estimate of sensible heat loss in watts. It does not tell you the greenhouse’s actual leakage rate.
Does a heater’s kW rating tell me its running cost?
No. A heater’s heat-output rating in kilowatts describes the rate at which it supplies heat; kilowatt-hours measure energy used over time. Running cost also depends on how long the heater operates, its actual energy input or fuel use, relevant efficiency and your local tariff. A thermostat may switch the heater off or reduce its operation between colder periods.
Should I add a fixed safety margin to the calculation?
There is no universal percentage that suits every greenhouse. First identify which inputs are measured, documented or uncertain, and which losses have been left out. If a cold excursion would have serious consequences, resolve the important unknowns or seek a qualified assessment rather than treating an arbitrary uplift as a guarantee.
The useful result is tied to one greenhouse, one overnight target and one cold-night assumption. Better measurements and documented thermal values narrow the uncertainty; a generic watt-per-square-metre rule cannot do that. Compare the stated peak with verified heater output, then use overnight temperature records to see whether the chosen setup is meeting its target in practice.
