Greenhouse heating needs vary with position,draught proofing and insulation. Generally 3kW will keep a 10’x 12’ frost free when -7°C outside. By installing Bubble Insulation the same heater will give higher temperatures, lower running costs or heat a larger greenhouse.
"Electric heaters are the most adaptable source of heating for smaller greenhouses and the most efficient means of maintaining specified temperatures.Thermostatic control ensures no wastage of heat resulting from changing external conditions.There are no fumes and no need for ventilation when the heaters are in use.Use of off-peak electricity,with an appropriate meter, will reduce running costs" RHS Website.