What is the most economical way to transition to LED ?


This blog posting provides more detailed explanation why Professor PawPower recommends changing to LED in a frequently used socket as soon as a bulb burns out and saving any existing stock of incandescent or CFL bulbs for sockets that are used less frequently. The suggested cutoff for “less frequently” is “less than an hour a day” on average.

LED bulbs are of course more energy efficient than incandescent or CFL ( compact fluorescent) bulbs.

But they also cost more.

 Naturally the more often and longer time a socket is used the faster is the payback for the more expensive bulb.

The main reason to change to LED is that they are more energy efficient, of course.

First we will take a look at just how much more economical LED bulbs are relative to CFL and incandescent.

The benefit we receive from any bulb is light output for a given period of time. There are two cost elements: the cost of the bulb itself (an investment) and the cost of operating the bulb ( cost of electricity to provide the light output).

We will calculate both cost elements for 1million lumen hours, which is just a little more than the benefit we receive from operating a regular 60W incandescent bulb over its lifetime. ( A 60 W standard incandescent bulb provides 860lumens light output for 1000 hours, so total 860,000 lumen hours.)

While many of the parameters may differ in any given situation, these are typical values.

This table clearly illustrates that an LED is providing light at a much lower cost – including the cost of the bulb itself.

The initial relatively high price we have to pay to purchase the LED bulb is “paid back” to us slowly in the form of lower operating costs as they consume less electricity.

This payback time of course depends on how often we operate that light. Some light sockets are operated rarely and for short time ( such as a light in the attic, or a walk-in cabinet) while others are used every day for several hours ( main lights in a living room).

The payback time for an LED bulb depends on the average time the socket is used each day. 

As the graph shows, the payback time is just about 3 years if a socket is used about 1 hour/day on average.

In general it is considered worthwhile to invest with a payback of 3 years or less.

It follows that while LED’s are consuming less electricity, if a socket is used infrequently the payback may be very long. In other words the inefficient incandescent still doesn’t cost much to operate per year because they are not used much in such “low usage” sockets.


And this leads to the original question “ what is the best strategy to transition to LED”

What do we do with the existing stock of incandescent and CFL bulbs? Use them up as bulbs burn out no matter where or is there a better way ?


To illustrate this we will calculate the operating costs for five years of a simplified situation. Let's assume a house has 8 “high use” sockets that are used on average 3 hours/day and another 8 “low use” sockets that are used just about 20 minutes per day. All the sockets are using incandescent bulbs that are “halfway” in their lifetime and there is a an existing stock of 8 incandescent lightbulbs.

We will compare two startegies: one being to use up the existing stock as the bulbs burn out and only when the stock is used up starting to buy LED-s. The other strategy is to use the stock of bulsb only for the “low use” sockets where the payback time for the LED is long. 

As the table shows, employing the strategy of saving the existing incandescent stock for the “low use” sockets generates savings that will approximately cover the cost of the 8 LED bulbs that need to be purchased even before the existing incandescent stock is depleted.  However, retrofitting all the sockets in the house would not pay off for a long time because the “low use” sockets generate savings too slowly to pay back the initial high cost of LED-s.

The savings generated by this strategy of course depends on the average time a "low use" socket is used daily, if it is approaching 1 hour/day instead of 20 minutes per day then the savings will be just about half as much. 

The electricity cost saving generated by the "high use" sockets is the reason that the recommended strategy is to use the existing stock of incandescent or CFL bulbs in “low use” sockets and to purchase LED-s for the “high use” sockets.



Prof. PawPower