During summertime the bats usually come out every night in search of food. During the day the bats will go into a state of torpor, saving energy. In summer the air temperatures during the day are not that cold so that the bats do not have to drop their temperatures much to reach a state of torpor. In wintertime, bats will stay in a state of torpor for days or even weeks. Only coming out on warmer nights in search of food.
Bats tend to live together socially in roosts.
Often there can be dozens of bats in the one roost. This makes it harder for them to drop their body temperature since the bats tend to keep each other warm. Hence the bats roosting together use more energy, as they can not drop their temperature as much as if they were on their own. Bats roosting on their own tend to get the greatest benefit from torpor.