Heat Mat for One Gallon Kombucha Home Brew This 5-watt heat mat slides perfectly underneath your 1 Gallon Brew Jar keeping it nice and warm...but not too warm! Plugs in, uses little energy, ensures a perfect kombucha batch. Monitor the temperature for the first 24 hours to ensure that you have struck the right balance. If need be, you can put the whole setup in the box that your Kombucha Home Brew Kit came it for another layer of insulation! We use our year round and have a perfect 78 degree brew going all the time.