Not sure if it would count as a bug, but it is a confusing change from the previous version: the Electrical system reports being Offline both on its display panel and on the Hab status displays (in red text on the Hab status screen) when it's turned on, has no component failures and has enough components to function, only it does not have sufficient power input, eg. during dusk/night/dawn.
In 0.57 this state was reported as something like "online but not meeting demand" in yellow, which was more descriptive of the state it is in. It is online, only it currently doesn't collect power but still serves the rest of the systems from the reserve batteries.
Customer support service by UserEcho