This is from a paper by Derek Mitchell who has done thermodynamic studies on bees
Q. Don’t bees use more energy when they are active? Colonies which are very well insulated don’t seem to cluster in winter – surely they will use up their stores too quickly?
A. No, they don’t use more energy. This is a misconception based on experiments on metabolic rate where bees were held in air at controlled temperatures. If they can warm the hive to a comfortable point, they don’t need much energy to stay alive – so the key is to provide an environment which they can warm at minimum cost. A well insulated cavity, in other words
The paper is on Springer so you have to pay for it but there is a taste of it here