Question

A 100 W light bulb has an efficiency of 95%. How much thermal energy will this light bulb add to the inside of a room in 2.5 h if the bulb is on for the whole time?

Answer 1

It seems like a very efficient light bulb, but assuming the given efficiency then 5% of the 100W will be given out as heat. So 5W = 5 J/s over 2.5 x 3600 s = 45kJ of Heat energy into the room. Of course the 95% of light energy may well be converted into heat when it interacts with a surface but that is probably another story :)