We like the Brewbelts ,if you want something easy and simple to keep your beer nice and warm then this is it.
You can move it up and down the container to alter the temperature a bit. The blurb that comes with says it's ok from 5 C to 24 C.
As a rough guide I'd say that if you're in shirt sleeves then turn it off, you don't need any heat!
If the house is very cold you might need to put a cover over the top of the fermenter to keep it warm- You need to use as thermometer at this point because the wine can get too hot when you do this-The stick on thermometers are ideal for this!