Ever wonder how much a CFL bulb will save you but didn't have a calculator handy? Well here's a simple and easy to remember rough calculation. For every 1 incandescent watt, at an average of 4 hours per day, it's about 1 cent per month savings. So replacing 100 watt bulb you use 4 hours per day saves $1.00 per month. 8 hours is $2.00 per month. 24 hours is $6.00 per month.
The only variable assumption here is 11 cents per kilowatt hour, so it's only an approximation, but still handy considering the ease at which you can apply it.
365 days / 12 months * 4 hours * 11 cents per kilowatt hour / 1000 watts * .75 = 1.00375 cents
Where .75 is the average CFL to Incandescent efficiency ratio (100 watt to 25 watts, 60 watts to 15 watts)