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.

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.

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.

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.