Greenhouse Heating Calculator (Heater Size & Cost)

Greenhouse Heating Guide

Why Greenhouse Heating Is Needed?

A greenhouse traps solar warmth by day, but on cold nights it loses heat rapidly through its large glazed surface — glass and thin polythene are poor insulators. Without heating, the inside temperature can fall close to the outside temperature overnight, so an unheated greenhouse offers only a few degrees of frost protection at best. Heating lets you: keep the greenhouse frost-free to protect plants from freezing (the most common goal, around 2°C minimum), overwinter tender plants that would die outdoors (a cool 5-7°C), or maintain active growth and propagation (a warm 10-13°C+). The colder the target relative to outside, and the larger and less-insulated the greenhouse, the more heat (and cost) is required. The principle is straightforward: the heater must replace heat as fast as the greenhouse loses it, to hold the target temperature on the coldest nights you want to protect against. This calculator estimates that peak heat loss and the heater size to match it, plus the running cost — which can be significant, so it's worth knowing before committing to a target temperature.

What do I need to know about Calculating Heat Loss?

Heat loss depends on three things: the surface area of glazing, how cold it is outside relative to your target inside temperature, and how well the glazing insulates. The calculation multiplies the total glazed surface area by the temperature difference (target inside minus coldest outside) and by a heat-loss factor for the glazing type (its U-value). Glazing matters greatly: single glass or single polythene loses heat fastest (high U-value). Twin-wall polycarbonate insulates considerably better (often cutting heat loss by a third or more). Adding bubble-wrap insulation inside glass is a cheap, effective way to reduce loss substantially. Temperature difference drives everything: holding 13°C inside when it's -5°C outside (an 18°C difference) requires far more heat than holding 2°C frost-free against the same -5°C (a 7°C difference). This is why frost-free protection is dramatically cheaper than maintaining a warm greenhouse. Design for your coldest realistic night: size the heater for the lowest temperature you actually want to protect against, not the average — but be realistic, as designing for extreme rare cold means a larger, costlier heater that's oversized most of the time.

What should I know about Heater Types and Sizing?

Greenhouse heaters come in several types. Electric fan heaters: common, responsive, and circulate air (reducing damp and fungal disease). Thermostatically controlled models only run when needed, which saves money. Clean (no fumes) but require a power supply and the running cost depends on electricity prices. Electric tubular heaters: gentle background heat, good for frost protection, often run along the base. Paraffin heaters: no electricity needed, useful where there's no power, but they produce water vapour and fumes (need ventilation) and are less controllable. Gas heaters: powerful but also produce moisture and fumes. For most home greenhouses, a thermostatically-controlled electric fan heater is the practical choice — sized to the calculated peak heat loss, with the thermostat ensuring it only runs when the temperature drops to the set point. Size the heater to meet or slightly exceed the peak heat loss figure; an undersized heater can't hold temperature on the coldest nights, while a thermostat prevents an adequately-sized one from wasting energy in milder conditions. This calculator recommends a heater wattage based on your greenhouse and target.

What do I need to know about Reducing Heating Costs?

Heating a greenhouse through winter can be surprisingly expensive, so reducing heat loss is usually more cost-effective than simply buying a bigger heater. Insulate: line the inside with horticultural bubble wrap (large-bubble type) — a cheap, very effective way to cut heat loss, often by a third. It reduces light slightly, a worthwhile trade in winter. Heat only what you need: rather than heating the whole greenhouse, use a heated propagator, a partitioned-off small area, or fleece over plants for localised warmth — far cheaper than maintaining the whole volume warm. Set the lowest viable temperature: frost-free (2°C) costs a fraction of maintaining 13°C. Only target higher temperatures for plants that genuinely need them. Use a thermostat: ensures the heater runs only when necessary — essential for economy. Reduce draughts: seal gaps, repair broken panes, and fit door/vent seals; draughts dramatically increase heat loss. Thermal mass: water barrels or stone absorb daytime warmth and release it at night, smoothing temperature swings. Position and shelter: a sheltered spot loses less heat to wind. Group plants together and use horticultural fleece on the coldest nights as extra protection. This calculator estimates both the heater size and the running cost, so you can weigh the cost of a given target temperature against insulating to bring that cost down.

Does Insulating a Greenhouse Reduce Heating Demand Significantly?

Greenhouses are inherently poor at retaining heat compared with a conventional building, since glass or polycarbonate glazing has a much higher rate of heat loss than an insulated wall, but several insulation measures can meaningfully reduce how much supplementary heating is needed to maintain a target temperature. Bubble wrap insulation, specifically horticultural-grade bubble insulation applied to the inside of glazing, is one of the most common and cost-effective measures, trapping a layer of still air against the glass that reduces heat loss by a meaningful margin while still allowing enough light through for plants, though it does reduce light transmission somewhat, which matters more for light-hungry plants than for general frost protection. Sealing gaps and improving the fit of doors and vents reduces draughts, which can account for a disproportionate share of heat loss in an older or poorly-maintained greenhouse structure, since draughts bypass whatever insulation is present entirely. Thermal mass — adding large containers of water, which absorb heat during the day and release it slowly overnight — is a passive measure that smooths out temperature swings and reduces how hard a heater needs to work to maintain a minimum overnight temperature, particularly effective in a greenhouse used primarily for frost protection rather than active growing. None of these measures eliminate the need for supplementary heating in cold climates, but combined, they can meaningfully reduce the heater size and running cost needed to maintain a given target temperature.

Frost Protection vs Year-Round Growing: Different Temperature Targets

The heating requirement for a greenhouse varies enormously depending on the goal, and conflating "greenhouse heating" as a single need leads to significant over- or under-sizing of the heater. Frost protection — simply keeping the greenhouse above freezing to protect overwintering plants, tender perennials, or an empty greenhouse's structure and contents from frost damage — requires maintaining a minimum temperature of only around 2-5°C, which is a relatively modest heating demand that many smaller heaters or even a well-insulated, thermal-mass-equipped unheated greenhouse can achieve through most of a typical UK winter with only occasional supplementary heat on the coldest nights. Year-round growing of tender or tropical plants, by contrast, often requires maintaining temperatures of 15-18°C or higher continuously through the coldest months, which represents a dramatically larger heating demand — potentially several times the heater output and running cost of simple frost protection — because the temperature differential being maintained against outside conditions is much greater, and because this higher target needs to be sustained around the clock rather than only during occasional cold snaps. Many greenhouse owners find a middle ground more practical and affordable: maintaining a moderate minimum temperature (around 7-10°C) sufficient for a wider range of plants to survive and even grow slowly through winter, without the substantially higher cost of true tropical year-round heating. Deciding which target applies before sizing a heater avoids both an underpowered heater that can't maintain true growing temperatures and an oversized, unnecessarily costly system for simple frost protection.

Worked Example: Heating Cost for a Small Greenhouse Overwinter

An 8ft × 6ft (roughly 4.5m²) glass greenhouse needs frost protection through a typical UK winter, keeping the internal temperature from dropping below around 5°C to protect overwintering tender plants. On a cold night with an external temperature of -2°C, maintaining a 7°C internal-to-external differential across typical single-glazed glass (a poor insulator, with a heat loss rate significantly higher than double glazing) requires a heater output of roughly 1-1.5kW for a greenhouse this size. Running an electric heater at 1.2kW for an average of 8 hours a night (the coldest overnight period) across a 90-night cold season uses roughly 1.2kW × 8 hours × 90 nights = 864 kWh. At a typical electricity price of around 25-28p/kWh, that's approximately £220-£240 for the season — a cost many growers reduce substantially by adding bubble-wrap insulation to the greenhouse panels (cutting heat loss by up to 50%) and using a thermostatically controlled heater that only runs when genuinely needed rather than continuously, which is why the running-cost range for frost protection on an equivalent greenhouse varies so widely in practice between well-insulated and uninsulated setups.