***Update - 10/4/09 - I have an improving lighting cost comparison that includes a lighting calculator and includes a residential LED bulb. See the incandescent, CFL, and LED lighting cost comparison and calculator.
In the last few months I've heard this statement twice: "I want to conserve energy. I plan on changing out my incandescent bulbs to CFLs as soon as they burn out."
While thinking of CFLs is a great start, there is no need to wait until your old bulbs burn out. If you do a life-cycle analysis (which I have done below) it makes much more sense to replace the incandescent bulbs immediately! This is not just from a "save the Earth" perspective, but especially a cost savings one.
Based on the cost of electricity at .09 cents/kwh, a regular old 60 watt light bulb would cost you $10.80 over its 2,000 hr life. The 14 watt CFL equivalent's electricity cost would only be $2.34 over the same 2,000 hrs (plus the CFL will last another 4,000 hrs after that)! The only way it would not make sense for you to immediately change that old bulb is if the CFL cost $8.46 or more, which I can assure you they don't ( I get mine for around $1.50 a pop). So what are you waiting for?
The figure below displays 3 different scenarios for a 10-year lighting analysis. The light is assumed to stay on for 4 hours a day, 365 days/year, for 10 years. The first case is all incandescent, the second case is all CFL, and the third case is you replace the incandescent after it burns out, and use CFLs after that. As you can see, immediately replacing the regular bulbs with CFLs provides the best payback!
-click on the thumbnail below, then move it around the screen if necessary