Which method you choose depends largely depend on your budget and personal preferences. The Royal Horticulture Society (RHS), Britain's leading gardening charity, recommends electric sources, even though they are costly, because they offer close control over the temperature and do not use combustion products, which can harm plants. Electric options include a fan heater, or a ground-source heat pump. Less expensive options include gas and oil systems such as boilers. But you must ensure proper ventilation, or your plants could be harmed. Other possibilities are solar panels, hot beds, and heated propagators.