For home brewers - don't pay stupid prices (up to £50 at Tesco Direct) to home brew outlets for an immersion heater. Go to allpondsolutions.co.uk who sell the same thing from £8.99 for a 50 watts, adjustable temperature (up to 32C) heater, meant for fish keepers. But they are the same thing. Various wattages eg. 100W, for not much more. Especially useful in winter as the thermostat keeps the temperature even which is important for a quick fermentation. More efficient than mats or belts where a lot of the heat is dissipated into the air..