Bees actually stop moving below 8degC so it's vital that they keep their home above that temperature. In practice bees will keep themselves clustered together on the combs at about 20degC. They don't generate this warmth by lighting little stoves and fuelling them with honey. They have a much more ingenious solution. They generate this heat by vibrating their biggest muscles, which are their wing muscles, very fast. You may be thinking that thousands of bees flapping their wings would surely also create a chilly breeze cooling the hive rather than heating it. Well it would but the bees have a trick up their sleeve. They are actually able to dislocate their wings so that they can vibrate the muscle without flapping their wings. Amazing!
The beehaus is triple insulated so that radiated heat losses are kept to a minimum. The more of the warmth that your bees can retain the more likely it is that they will come through even hard winters without running out of fuel and will be in good condition the following spring.
The bees will need 20kg of honey to see them comfortably through the winter. If the bees have stored 30kg you can remove 10kg without any consequence. However, if you need to take 15kg of honey because you've promised all your friends, neighbours and aunts and uncles a jar for Christmas you will need to make up the 5kg deficit.
You do this by feeding the bees the equivalent weight in sugar as a syrup. This is quite simple to do and the bees will store it in the cells exactly as they would if it were honey.
The time to do this is in September and is simply done by placing the fondant on top of the clearer board with the bee escape removed. The bees will quickly move the sugary liquid into the cells in the middle of the combs and seal it.
There is more about feeding bees in the beekeeping basics section.