Answer
a. It is not wasteful.
b. It is wasteful.
Work Step by Step
a. Light bulbs convert electrical energy to thermal energy with perfect efficiency (the light that the bulb emits is eventually absorbed by the walls, floor, and ceiling, heating them up). Regular resistance heating (the kind where electricity runs through a wire and heats it up) does the same thing.
Turning all the lights on just means that they do the work of the heaters. The electric bill is the same.
b. On the other hand, if the building is being cooled by air conditioners, it is wasteful to keep all the lights on. The lights generate heat, and the air conditioners must work harder to remove their excess thermal energy.
