Heating a greenhouse in winter starts with the plants you want to keep and the lowest temperature they can tolerate. A heater is only one part of the decision: the structure, thermostat, electricity supply and overnight running cost all need to suit the job.
This guide compares the main approaches and shows how to calculate the cost from your own electricity rate. It is a researched planning guide, not a hands-on heater test or a promise that a particular wattage will protect every greenhouse.
Table of Contents
Decide what you actually need to heat
Make a short list of the plants, their required minimum temperatures and the space they occupy. Some can stay in an unheated structure; others need a warmer location. Our guide to overwintering plants in an unheated greenhouse helps with that first decision.
The RHS greenhouse-heating guide describes maintaining around 4°C as a frost-free approach for a wider range of tender plants. That is not a universal setting: check each plant’s requirements. A thermostat and an independent minimum/maximum thermometer near the plants help you check the temperature they experience.
Separate two jobs on your list: keeping established plants through winter and warming seed compost for germination. If only a few trays need heat, compare heated and unheated propagators before budgeting to warm the entire building.
Compare the heating options by the job
| Approach | Where to consider it | What to check |
|---|---|---|
| Greenhouse electric fan heater | A controlled source of heat with air circulation. | Suitable electrical supply, thermostat, permitted clearances and the manufacturer’s sizing assumptions. |
| Greenhouse tube heater | Background warmth where a modest heat input may be sufficient. | Whether the output actually meets the heat loss; low wattage alone does not demonstrate adequate frost protection. |
| Heated propagator | A defined group of seed trays requiring warmth. | Tray dimensions, temperature control and the surrounding light and air conditions. |
| Purpose-made gas or paraffin greenhouse heater | A site without a suitable mains connection. | Fuel storage, the exact appliance instructions and required ventilation; include these in the decision before buying. |
The RHS distinguishes fan heaters from lower-output background tube heating. It also warns that gas and paraffin combustion add moisture and combustion products, including carbon monoxide, requiring ventilation. Repairing glazing and poorly fitting doors or vents can reduce heat loss, but preserve usable ventilation. See our guide to replacement glazing clips and seals before ordering parts.
How much does a greenhouse heater cost to run?
Use the heater’s electrical input and your tariff’s unit rate. A kilowatt is 1,000 watts; one kilowatt-hour is the energy used by a 1 kW load running for one hour. These units are explained in the Energy Saving Trust glossary.
Running cost = input in kW × hours operating at that input × electricity price per kWh.
The examples below use an illustrative rate of 30p per kWh. This is not a quote for the current price cap or your tariff. Each row assumes the stated load operates continuously for eight hours; it does not predict a thermostat-controlled night’s consumption.
| Illustrative electrical load | Energy over eight hours | Cost at 30p/kWh |
|---|---|---|
| 150 W (0.15 kW) | 1.2 kWh | £0.36 |
| 500 W (0.5 kW) | 4 kWh | £1.20 |
| 2,000 W (2 kW) | 16 kWh | £4.80 |
If a 2 kW heating element runs for four of those eight hours, its heating cost at the example rate is £2.40. Add any separate fan or standby use. For a variable-rate tariff, calculate each time band separately. These are our arithmetic examples, not measured product running costs.
Do not compare the first and last rows as if they deliver the same result. A small heater running continuously may still leave the greenhouse too cold. A larger one may cycle, or may also run continuously in severe conditions. Ask a supplier to size for your greenhouse dimensions, glazing, exposure and target temperature at the expected outdoor minimum; a headline floor-area rating without those assumptions is incomplete.
Understand what the thermostat controls
Check whether a product has an integrated thermostat, a separate plug-in controller or no automatic temperature control. Also check whether switching off the heating element stops the fan. These arrangements affect operation and energy use.
For example, the manufacturer’s Bio Green Palma 2,000 W page describes a version supplied with a separate digital thermostat and a fan-only setting. That illustrates why the bundle and operating modes matter; it is not a UK stock or price recommendation. Read the manual for the exact version being sold.
Use your minimum/maximum greenhouse thermometer to record overnight results. Keep a simple log of the setting, observed minimum, weather and energy use where suitable metering is available. This gives you evidence for adjustments rather than assuming that the number on the dial describes every corner.
Can a solar-powered heater protect plants overnight?
Solar electricity can contribute to heating, but an overnight frost-protection plan needs an energy source after sunset. The Centre for Sustainable Energy explains that photovoltaic panels generate nothing at night and that batteries can store daytime production for later use. A panel and a small ventilation fan do not establish that a system supplies useful heat overnight.
The Energy Saving Trust’s solar-power guide also notes lower solar generation in winter. Before accepting a solar-heater claim, ask for the actual heat output, usable stored energy, winter generation assumptions and backup arrangement for successive dull days.
As a scale check, a 500 W load operating for ten hours needs 5 kWh of delivered electricity. Storage and conversion losses would increase the energy required upstream. This is an illustrative energy calculation, not a battery-system design: have an installer assess the complete system and its reserve before relying on it for frost protection.
Plan the electrical supply and clearances first
Have the greenhouse electrical connection installed professionally, as the RHS advises. Choose equipment explicitly suitable for the greenhouse environment and follow its installation instructions. Check the reach of the supplied cable, moisture protection, controller compatibility and the required clear space around the heater before buying; do not improvise wiring or assume an indoor room heater is suitable.
Electrical Safety First says outdoor sockets supplying equipment should have RCD protection and warns against using patio heaters indoors. A patio heater is not a substitute for a greenhouse-rated appliance in an enclosed structure. Keep combustible covers, packaging and stored materials away as the appliance manual requires.
Manage damp as well as temperature
A greenhouse still needs ventilation. The RHS ventilation guidance explains how air exchange helps control heat and fungal problems. Leave doors and vents usable when adding insulation, and respond to the conditions rather than permanently sealing the structure.
Use our greenhouse ventilation guide to plan airflow and the condensation guide to investigate persistent damp. A useful winter setup is one you can inspect, measure and adjust, with a plan to move vulnerable plants if the heating fails.
Sources checked 25 October 2026. Cost examples are calculated illustrations; use your own tariff and the instructions for your exact appliance.