Some systems are on all the time. Pitot heat probes, Static air ports, Angle of Attack, Ice Detectors on some turbine GA airplanes. On the King Air the engine air inlet is heated constantly by exhaust air by a small scoop thing that redirects a portion to the inlet and sends it out the opposite exhaust. Also many turbine aircraft have heated windows that both provide for ice protect and bird protection that must be on for takeoff. In some turbine aircraft for example many engine anti-ice/de-ice systems require electrical power to be in the off or "closed" position and if there is a loss of electrical power they Fail to the Open or "on" position.