CFL Light Bulbs ROI
Let's first take a look at a typical home fixture for general lighting. Let's say you currently have a 60-watt incandescent and that you use it six hours a day. The average U.S. home electricity price is 12 cents per kWh, according to the most recent data from the U.S. Energy Information Administration. (If you live in a state with appreciably different electricity rates, you should adjust your own calculations accordingly.) So, if you pay 12 cents per kWh, and that light bulb uses 60 watts x 6 hours x 365 = 131.4 kWh a year, that's costing you $16 a year.
Of course, you'll also have to buy the bulb itself, which isn't the most expensive part of the equation when it comes to lighting. If you buy them in a 6-pack, you can now get an Energy Star-rated, major brand CFL for $2 each (like the Philips CFL light bulbs pictured here). Get them in a two-pack from brands like GE for $3 each at your local big box. Regular old incandescents still go for around 75 cents each. But you'll need a lot more of them, since they only last around 1,000 hours, as opposed to around 10,000 hours for most good CFLs. If the light is used six hours a day, you'd go through two incandescents in one year, or 10 in five years. In the latter period, you'd spend $7.50 for 10 incandescents, versus $3 for one CFL.
If you put a 13-watt CFL in the fixture, you'll get the same amount of light, but only spend 13 watts x 6 hours x 365 = 28.5 kWh a year, which costs nearly $3.40. So you save $12.60 a year versus having an incandescent. Divide that by 12 to get a monthly average of just over $1. That means the payback period for a light bulb that is between two and three dollars more expensive than an incandescent upfront is somewhere between two and three months.
Multiply that result by how many general use fixtures you have in your house to see the savings grow. Of course, not all fixtures are general. But there are now good CFL replacements for several key areas, including bathroom vanities, candelabra-style fixtures, outdoor flood lights, outdoor post lamps, and even ceiling fan fixtures (avoid putting a regular CFL in a fan, because the vibrations can shorten its life and increase risk of breakage).
Even so, there are still some places where CFLs aren't the best. Rapid switching greatly reduces their lifespan, so you may not want to use them in areas where you will be turning them off frequently after short periods. For many of us, this means they may not be the best choice for closets, halls and rooms we rarely use. Instead, you may want to rotate through old incandescents to "use them up," or read below about halogens.