I keep quite a cool house because I work outdoors and don't like overheated buildings. It follows that I needed to find a way to warm my wine fermenting demijohn. Since I already had a water distiller which the demijohn fitted nicely into I did the maths and worked out that a 600W distiller would give out 6W of heat when run at a tenth of its design voltage. In this case a 240V distiller running on a 24V transformer. For 120V you would need to use a 12V transformer. 6 Watts might not sound a lot, but a distiller has a thermally isolated chamber and if a folded length of paper towel is stuffed in around the top of the demijohn to avoid cooling airflow it creates an efficient way to heat the demijohn and its contents. In a cold house at about 12 degrees centigrade the wine was held at a temperature of about 25 degrees centigrade and fermented well.
If you choose to try this yourself then I'd suggest using a plug-in transformer with an AC output with a rating of about 12VA. I used a Christmas lighting transformer. Make sure you don't accidentally use the distiller directly on mains power with a demijohn in it as the full 600W power would kill the yeast and possibly damage the demijohn.