When a bat is sitting on the power line and not touching anymore wires except just one wire then they won't get electrocuted.
But when a bats wings make contact with more than one wire on the power lines then that's when they get electrocuted and they usually just hang onto the wires while being electrocuted because both wings are touching the wires so they cannot fall to the ground like a bird could that doesn't have as big of wings.
For electrocution to happen the bird or bat has to make contact with the hot wire and the neutral or ground wire and if they do not make contact with both then they won't be electrocuted.
However if the bat or bird touches both wires at the same time then the electrocution starts and they become electrocuted.