The other day the light bulb on my porch finally gave up the ghost and so I ran down to my favorite hardware store to pick up a new one. When I got there I was presented with a choice that many people struggle with: is it better to buy the regular old incandescent bulb or spend significantly more money for the energy efficient compact fluorescent light (CFL) instead?
In my case the CFL, priced at $7.50, was exactly six times more expensive than the incandescent bulb priced at $1.25! With a price difference like that it is not hard to see why many people are loathe to make the conversion to CFLs.
The big question, of course, was just how quickly would it take the energy savings provided by the CFL to cover the product’s purchase price?
How to Determine the Payback Period of Energy Efficient Bulbs
To answer that, one has to first determine the payback period for purchasing the more expensive CFL. So just how does a person go about doing that? The answer is to calculate the simple payback period, which in this case is defined as:
Simple Payback = (CFL Cost) / (Annual Electricity Savings)
By calculating simple payback you can easily determine just how long it will take to show a return on your investment in the more expensive CFLs. Although it’s true that simple payback does not take into account inflation rates, compounded savings, or replacement costs, it is a very useful tool nevertheless and more than adequate for this particular purpose.
Looking at the simple payback equation, all one needs to know is the cost of the CFL and the annual electricity savings. I found the former by simply looking at the price tag, but to get the latter I had to know two things: 1) how many hours per day the light was in use; and, 2) the number of watts saved by going from an incandescent bulb to a CFL.
For the first point, I conservatively assumed my porch light was on 2 hours per day – it’s on much less. To determine the second point, I simply looked at the labeled wattage ratings of the two bulbs; in my case, the incandescent bulb was rated at 60 Watts while the CFL’s rating was listed at a miserly 13 Watts, for a total power savings of 47 Watts.
Calculating Your Annual Energy Savings
With that information in hand, I was able to calculate my annual energy savings using this formula:
Annual Electricity Savings = ((Daily Hours Used x 365 days/year) x (Watts Saved))/1000 x (Cost of Electricity)
In my case, plugging in the numbers looked like this:
Annual Electricity Savings = ((2 hrs x 365 days/yr) x (47 Watts))/1000 x ($0.15/kWh)
…for an annual savings of $5.15.
Calculating the Payback Period
With our annual energy savings figured out, we can now calculate the payback. If the energy efficient product costs $7.50, and the annual electricity savings is $5.15 as shown above, the simple payback is:
$7.50/$5.15 = 1.46 years
It is interesting to note that if you intend on replacing all of the bulbs in your home with CFLs, increasing the number of bulbs being replaced does not decrease the payback period (assuming there are no economies of scale for buying additional bulbs); in my case it would still be 1.46 years. It will, however, increase your annual energy savings.
Evaluating the Payback Period
As a rule of thumb, payback periods of five years or less are often generally worth implementing, while projects with payback periods greater than ten years are generally not cost effective. In my case, the 1.46 year payback period made the decision to go with the CFL an easy one.
That doesn’t mean I am going to immediately replace all of my incandescent bulbs with CFLs, but I will begin transitioning to CFLs as my incandescent bulbs burn out. 
And For the Mathematically Challenged…
By the way, if you don’t feel like doing the long-hand calculations yourself, there are also handy web-based savings calculators that can be used to determine the payback periods and realized annual savings from using CFLs instead of incandescent bulbs. 
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CFLs are absolutely worth it! I’ve been using the same one in my living room for over 6 years and it’s been on every day from 6-14 hrs/day.
Wow! If I did my math correctly, and assuming an average of 10 hrs per day, that’s almost 20,000 hrs of operation! Incredible!
Shouldn’t the numerator be CFL – incandescent cost since that is the increased investment of purchasing a CFL?
John,
It’s a very very fine distinction, but ’simple payback’ only considers the amount of time required for the benefits to pay back the cost of a project. For that reason the numerator should not include the subtracted cost of the incandescent bulb. Ticky tack, I know.
Len
Definitely worth the initial cost of $5 or $6, considering the amount of incandescents I would have used over 6 years. It’s also a 3-way bulb and just recently moved with me from Virginia to Georgia, and still going strong.
That’s awesome, Jennifer. I’m beginning to think I should be including gasoline savings from the reduced number of trips required to buy new incandescent bulbs too…
In my home the porch light may very well be on all night as I like the place lit up. If not all night – at least 4 -5 hours. We have made the switch in our home and are looking forward to the savings.
Good for you, Sadie! I’m the opposite of you, though. I like the darkness, so my porch light is set on a motion detector. As a result, I’ll bet my new CFL porch light will last for close to a thousand years.
Great article! I know you were not considering it in your calculation, but CFLs also last 8 or more times longer than incandescents bulbs. So if you just figure on replacement cost alone, not electricity savings, (and excluding inflation, etc etc), you will save money over the lifetime of the CFL (1 CFL=$7.50, 8 incandescents bulbs= $10).
Hey, thanks, Emily!
And you are absolutely correct – and Jennifer’s CFLs are lasting even longer than that!
Len
CFL’s seem to be more resilient to power spikes etc. I haven’t had one blow from a power spike/lightning strike yet, but those event’s did successfully claim the lifes of several incandescent bulbs, nothing but CFL’s in the house now.
For living rooms use the warm whites for things like desklamps were you work by the light try cold whites(especially if you draw/paint)
In the philippines, a brand name 9 watt CFL like GE or phillips should cost around us$ 2.12. A chinese no-name brand maybe half that. Of course the chinese brands tend to die much earlier, maybe 1-2 years in my experience.
@S01: I just replaced two CFLs in two of my bathrooms that were almost 13 years old. They do last a long long time under the right conditions.
It’s truly great to see a cool blogpost , thx.
Keep in mind for indoor applications you have to consider in winter incandescents will reduce heat required, and in summer, if A/C is used will more then double the energy wasted, 75 watts of heat wasted may mean 150-300 watts of A/C usage to remove that heat
So then I guess for those of you who want to be as green as possible, use CFLs in the summertime and incandescent bulbs in the winter time.
Even in the winter, electric resistance heat (an incandescent bulb) is an expensive way to heat.
I’d rather spend a kilowatt-hour on my heat pump to get 3 kW-hour equivalent (or use the gas furnace if the temperature is below freezing)
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