You answer is best described like a lwyer would answer it; it depends.
What type of light bulb are you replacing? How long is this bulb actually on during the day? What are you replacing it with and can you use a smaller bulb (less illumination)?
For a general answer the incandescent bulb waste nearly 90 percent of its input wattage. This waste is heat and thus is the reason the buld is so hot when it is on for even a short period. Fluorescent have a much better efficacy (lumen to watt ratio), generate much less waste heat and are affordable now. The LED lamps have an even better efficacy and produce nearly no heat. Unfortunately they remain cost prohibitive but this fact will change very soon and these will be the lamps of the next generation.